feat(public-api): T36-T38 implement public API endpoints

- GET /api/v1/stats: aggregated stats with date range (default 30 days)
- GET /api/v1/usage: paginated usage with required date filters
- GET /api/v1/keys: key list with stats, no key values exposed
- All endpoints use API token auth and rate limiting

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 public_api
 from openrouter_monitor.routers import stats
 
 settings = get_settings()
@@ -33,6 +34,7 @@ app.add_middleware(
 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.include_router(public_api.router)
 
 
 @app.get("/")