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/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'] );
+	}
+}