feat(stats): T32-T33 implement dashboard and usage endpoints

Add statistics router with two endpoints:
- GET /api/stats/dashboard: Aggregated dashboard statistics
  - Query param: days (1-365, default 30)
  - Auth required
  - Returns DashboardResponse

- GET /api/usage: Detailed usage statistics with filtering
  - Required params: start_date, end_date
  - Optional filters: api_key_id, model
  - Pagination: skip, limit (max 1000)
  - Auth required
  - Returns List[UsageStatsResponse]

Also add get_usage_stats() service function for querying
individual usage records with filtering and pagination.

src/openrouter_monitor/main.py

@@ -8,6 +8,7 @@ from fastapi.middleware.cors import CORSMiddleware
 from openrouter_monitor.config import get_settings
 from openrouter_monitor.routers import api_keys
 from openrouter_monitor.routers import auth
+from openrouter_monitor.routers import stats
 
 settings = get_settings()
 
@@ -31,6 +32,7 @@ app.add_middleware(
 # Include routers
 app.include_router(auth.router, prefix="/api/auth", tags=["authentication"])
 app.include_router(api_keys.router, prefix="/api/keys", tags=["api-keys"])
+app.include_router(stats.router)
 
 
 @app.get("/")