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>

.gitignore

@@ -16,6 +16,11 @@ wp-content/uploads/
 node_modules/
 vendor/
 
+# Environment & credentials
+.env
+.env.local
+.env.credentials
+
 # Logs
 *.log
 debug.log