feat: add LLM cost tracking with budget enforcement and admin dashboard

- New Cost_Tracker class: logs every API call (model, tokens, cost), enforces
  monthly budgets with hard-stop mode, provides aggregated reporting
- New llm_costs database table with indexes on created_at, model, operation
- Modified LLM class: call_openrouter_with_usage() extracts token usage from
  OpenRouter responses; budget gate checks before each API call
- Admin settings: monthly budget (USD), enforcement mode (disabled/warn/hard-stop),
  cost log retention period
- Admin cost dashboard: budget progress bar, daily cost chart (Chart.js),
  per-model and per-operation breakdowns, recent API call log
- PHPUnit tests: 16 tests covering cost calculation, pricing lookups,
  budget enforcement logic, and model validation
- Updated ARCHITECTURE.md with new table schema, data flow, and key decisions
- Updated CONVENTIONS.md with "how to add cost tracking for a new operation"
- Version bump to 2.4.0

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/unit/CostTrackerTest.php

@@ -0,0 +1,213 @@
+<?php
+/**
+ * Unit tests for Peptide_News_Cost_Tracker.
+ *
+ * Tests cost calculation, budget enforcement logic, and model pricing lookups.
+ * Database-dependent methods (log_api_call, get_cost_summary, etc.) are covered
+ * by integration tests; these unit tests focus on pure logic.
+ *
+ * @since 2.4.0
+ */
+
+use PHPUnit\Framework\TestCase;
+
+class CostTrackerTest extends TestCase {
+
+	/**
+	 * Reset global test options before each test.
+	 */
+	protected function setUp(): void {
+		global $_test_options;
+		$_test_options = array();
+	}
+
+	// =============================================
+	// calculate_cost() — pricing calculation tests
+	// =============================================
+
+	public function test_calculate_cost_with_known_model(): void {
+		// google/gemini-2.0-flash-001: input $0.10/1M, output $0.40/1M
+		$cost = Peptide_News_Cost_Tracker::calculate_cost(
+			'google/gemini-2.0-flash-001',
+			1000, // 1K prompt tokens
+			500   // 500 completion tokens
+		);
+
+		// Expected: (1000/1M * 0.10) + (500/1M * 0.40) = 0.0001 + 0.0002 = 0.0003
+		$this->assertEqualsWithDelta( 0.0003, $cost, 0.000001 );
+	}
+
+	public function test_calculate_cost_with_free_model(): void {
+		$cost = Peptide_News_Cost_Tracker::calculate_cost(
+			'google/gemma-3-27b-it:free',
+			5000,
+			2000
+		);
+
+		$this->assertEqualsWithDelta( 0.0, $cost, 0.000001 );
+	}
+
+	public function test_calculate_cost_with_unknown_model_returns_zero(): void {
+		// Unknown models default to zero pricing to avoid overcharging estimates.
+		$cost = Peptide_News_Cost_Tracker::calculate_cost(
+			'unknown/model-xyz',
+			10000,
+			5000
+		);
+
+		$this->assertEqualsWithDelta( 0.0, $cost, 0.000001 );
+	}
+
+	public function test_calculate_cost_with_expensive_model(): void {
+		// anthropic/claude-3.5-sonnet: input $3.00/1M, output $15.00/1M
+		$cost = Peptide_News_Cost_Tracker::calculate_cost(
+			'anthropic/claude-3.5-sonnet',
+			2000,  // 2K prompt tokens
+			1000   // 1K completion tokens
+		);
+
+		// Expected: (2000/1M * 3.00) + (1000/1M * 15.00) = 0.006 + 0.015 = 0.021
+		$this->assertEqualsWithDelta( 0.021, $cost, 0.000001 );
+	}
+
+	public function test_calculate_cost_with_zero_tokens(): void {
+		$cost = Peptide_News_Cost_Tracker::calculate_cost(
+			'anthropic/claude-3.5-sonnet',
+			0,
+			0
+		);
+
+		$this->assertEqualsWithDelta( 0.0, $cost, 0.000001 );
+	}
+
+	public function test_calculate_cost_precision_with_large_token_count(): void {
+		// Simulate a large batch: 1M prompt tokens with gpt-4o ($2.50/1M)
+		$cost = Peptide_News_Cost_Tracker::calculate_cost(
+			'openai/gpt-4o',
+			1000000, // 1M prompt tokens
+			500000   // 500K completion tokens
+		);
+
+		// Expected: (1M/1M * 2.50) + (500K/1M * 10.00) = 2.50 + 5.00 = 7.50
+		$this->assertEqualsWithDelta( 7.5, $cost, 0.000001 );
+	}
+
+	// =============================================
+	// get_model_pricing() — pricing lookup tests
+	// =============================================
+
+	public function test_get_model_pricing_returns_known_defaults(): void {
+		$pricing = Peptide_News_Cost_Tracker::get_model_pricing( 'openai/gpt-4o-mini' );
+
+		$this->assertArrayHasKey( 'input', $pricing );
+		$this->assertArrayHasKey( 'output', $pricing );
+		$this->assertEqualsWithDelta( 0.15, $pricing['input'], 0.001 );
+		$this->assertEqualsWithDelta( 0.60, $pricing['output'], 0.001 );
+	}
+
+	public function test_get_model_pricing_unknown_model_returns_zero(): void {
+		$pricing = Peptide_News_Cost_Tracker::get_model_pricing( 'totally/unknown-model' );
+
+		$this->assertEqualsWithDelta( 0.0, $pricing['input'], 0.001 );
+		$this->assertEqualsWithDelta( 0.0, $pricing['output'], 0.001 );
+	}
+
+	public function test_get_model_pricing_respects_custom_overrides(): void {
+		global $_test_options;
+		$_test_options['peptide_news_custom_model_pricing'] = array(
+			'custom/my-model' => array( 'input' => 1.50, 'output' => 5.00 ),
+		);
+
+		$pricing = Peptide_News_Cost_Tracker::get_model_pricing( 'custom/my-model' );
+
+		$this->assertEqualsWithDelta( 1.50, $pricing['input'], 0.001 );
+		$this->assertEqualsWithDelta( 5.00, $pricing['output'], 0.001 );
+	}
+
+	public function test_custom_pricing_overrides_defaults(): void {
+		global $_test_options;
+		// Override the default pricing for gemini flash.
+		$_test_options['peptide_news_custom_model_pricing'] = array(
+			'google/gemini-2.0-flash-001' => array( 'input' => 0.50, 'output' => 1.00 ),
+		);
+
+		$pricing = Peptide_News_Cost_Tracker::get_model_pricing( 'google/gemini-2.0-flash-001' );
+
+		$this->assertEqualsWithDelta( 0.50, $pricing['input'], 0.001 );
+		$this->assertEqualsWithDelta( 1.00, $pricing['output'], 0.001 );
+	}
+
+	// =============================================
+	// is_budget_exceeded() — budget enforcement tests
+	// =============================================
+
+	public function test_budget_disabled_never_exceeds(): void {
+		global $_test_options;
+		$_test_options['peptide_news_budget_mode']    = 'disabled';
+		$_test_options['peptide_news_monthly_budget'] = 10.0;
+
+		$this->assertFalse( Peptide_News_Cost_Tracker::is_budget_exceeded() );
+	}
+
+	public function test_budget_zero_limit_never_exceeds(): void {
+		global $_test_options;
+		$_test_options['peptide_news_budget_mode']    = 'hard_stop';
+		$_test_options['peptide_news_monthly_budget'] = 0.0;
+
+		$this->assertFalse( Peptide_News_Cost_Tracker::is_budget_exceeded() );
+	}
+
+	public function test_budget_warn_only_never_exceeds(): void {
+		global $_test_options;
+		$_test_options['peptide_news_budget_mode']    = 'warn_only';
+		$_test_options['peptide_news_monthly_budget'] = 1.0;
+
+		// warn_only should never return true (it only logs warnings).
+		$this->assertFalse( Peptide_News_Cost_Tracker::is_budget_exceeded() );
+	}
+
+	// =============================================
+	// DEFAULT_MODEL_PRICING — structure validation
+	// =============================================
+
+	public function test_all_default_pricing_has_required_keys(): void {
+		foreach ( Peptide_News_Cost_Tracker::DEFAULT_MODEL_PRICING as $model => $pricing ) {
+			$this->assertArrayHasKey( 'input', $pricing, "Model {$model} missing 'input' key" );
+			$this->assertArrayHasKey( 'output', $pricing, "Model {$model} missing 'output' key" );
+			$this->assertIsFloat( $pricing['input'], "Model {$model} 'input' should be float" );
+			$this->assertIsFloat( $pricing['output'], "Model {$model} 'output' should be float" );
+			$this->assertGreaterThanOrEqual( 0.0, $pricing['input'], "Model {$model} 'input' should be non-negative" );
+			$this->assertGreaterThanOrEqual( 0.0, $pricing['output'], "Model {$model} 'output' should be non-negative" );
+		}
+	}
+
+	public function test_default_pricing_includes_key_models(): void {
+		$required_models = array(
+			'google/gemini-2.0-flash-001',
+			'anthropic/claude-3.5-sonnet',
+			'openai/gpt-4o-mini',
+		);
+
+		foreach ( $required_models as $model ) {
+			$this->assertArrayHasKey(
+				$model,
+				Peptide_News_Cost_Tracker::DEFAULT_MODEL_PRICING,
+				"Default pricing should include {$model}"
+			);
+		}
+	}
+
+	// =============================================
+	// Budget mode constants — sanity checks
+	// =============================================
+
+	public function test_budget_mode_constants_are_distinct(): void {
+		$modes = array(
+			Peptide_News_Cost_Tracker::BUDGET_MODE_HARD_STOP,
+			Peptide_News_Cost_Tracker::BUDGET_MODE_WARN_ONLY,
+			Peptide_News_Cost_Tracker::BUDGET_MODE_DISABLED,
+		);
+
+		$this->assertCount( 3, array_unique( $modes ), 'Budget mode constants must be distinct' );
+	}
+}

tests/unit/LlmUsageTest.php

@@ -0,0 +1,82 @@
+<?php
+/**
+ * Unit tests for LLM cost tracking integration.
+ *
+ * Tests that call_openrouter_with_usage() correctly structures its
+ * return array and that the backward-compatible call_openrouter()
+ * wrapper still works.
+ *
+ * @since 2.4.0
+ */
+
+use PHPUnit\Framework\TestCase;
+
+class LlmUsageTest extends TestCase {
+
+	protected function setUp(): void {
+		global $_test_options;
+		$_test_options = array();
+	}
+
+	// =============================================
+	// is_valid_model() — unchanged, regression check
+	// =============================================
+
+	public function test_valid_model_format(): void {
+		$this->assertTrue( Peptide_News_LLM::is_valid_model( 'google/gemini-2.0-flash-001' ) );
+		$this->assertTrue( Peptide_News_LLM::is_valid_model( 'anthropic/claude-3.5-sonnet' ) );
+		$this->assertTrue( Peptide_News_LLM::is_valid_model( 'google/gemma-3-27b-it:free' ) );
+	}
+
+	public function test_invalid_model_format(): void {
+		$this->assertFalse( Peptide_News_LLM::is_valid_model( '' ) );
+		$this->assertFalse( Peptide_News_LLM::is_valid_model( 'no-slash-model' ) );
+		$this->assertFalse( Peptide_News_LLM::is_valid_model( 'has spaces/model' ) );
+	}
+
+	// =============================================
+	// is_enabled() — budget-aware checks
+	// =============================================
+
+	public function test_is_enabled_false_when_no_api_key(): void {
+		global $_test_options;
+		$_test_options['peptide_news_llm_enabled']       = 1;
+		$_test_options['peptide_news_openrouter_api_key'] = '';
+
+		$this->assertFalse( Peptide_News_LLM::is_enabled() );
+	}
+
+	public function test_is_enabled_false_when_disabled(): void {
+		global $_test_options;
+		$_test_options['peptide_news_llm_enabled']       = 0;
+		$_test_options['peptide_news_openrouter_api_key'] = 'sk-test-key';
+
+		$this->assertFalse( Peptide_News_LLM::is_enabled() );
+	}
+
+	// =============================================
+	// process_article() — budget gate behavior
+	// =============================================
+
+	public function test_process_article_returns_early_when_disabled(): void {
+		global $_test_options;
+		$_test_options['peptide_news_llm_enabled']       = 0;
+		$_test_options['peptide_news_openrouter_api_key'] = '';
+
+		$article = (object) array(
+			'id'          => 1,
+			'title'       => 'Test Article',
+			'excerpt'     => 'Test excerpt.',
+			'content'     => 'Test content.',
+			'categories'  => 'test',
+			'tags'        => '',
+			'ai_summary'  => '',
+		);
+
+		$result = Peptide_News_LLM::process_article( $article );
+
+		$this->assertFalse( $result['success'] );
+		$this->assertEmpty( $result['keywords'] );
+		$this->assertEmpty( $result['summary'] );
+	}
+}