feat(api-keys): T24-T27 implement API keys CRUD endpoints

- T24: POST /api/keys with encryption and limit validation
- T25: GET /api/keys with pagination and sorting
- T26: PUT /api/keys/{id} for partial updates
- T27: DELETE /api/keys/{id} with cascade
- Add ownership verification (403 for unauthorized access)
- API key encryption with AES-256 before storage
- Never expose API key value in responses
- 100% coverage on api_keys router (25 tests)

Refs: T24 T25 T26 T27

src/openrouter_monitor/main.py

@@ -6,6 +6,7 @@ from fastapi import FastAPI
 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
 
 settings = get_settings()
@@ -29,6 +30,7 @@ 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.get("/")