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>

uninstall.php

@@ -1,13 +1,17 @@
 <?php
+declare( strict_types=1 );
+
 if ( ! defined( 'WP_UNINSTALL_PLUGIN' ) ) {
     exit;
 }
 global $wpdb;
 $prefix = $wpdb->prefix;
 
-// Drop custom tables.
+// Drop custom tables (order matters: child tables first due to FK constraints).
 $wpdb->query( "DROP TABLE IF EXISTS {$prefix}peptide_news_clicks" );
 $wpdb->query( "DROP TABLE IF EXISTS {$prefix}peptide_news_daily_stats" );
+$wpdb->query( "DROP TABLE IF EXISTS {$prefix}peptide_news_log" );
+$wpdb->query( "DROP TABLE IF EXISTS {$prefix}peptide_news_llm_costs" );
 $wpdb->query( "DROP TABLE IF EXISTS {$prefix}peptide_news_articles" );
 
 // Delete all plugin options.