feat: implement Phase 2 stabilization

- Cache Graph API tokens with expiry-aware reuse in graph/auth.py
- Add tenacity-based retry/backoff wrapper (utils/http.py) and apply to all Graph/source API calls
- Add Pydantic request/response models (models/api.py) and FastAPI query constraints
- Add unit tests for event_model, auth and integration tests for API endpoints
- Configure ruff linter/formatter in pyproject.toml
- Add GitHub Actions CI pipeline (.github/workflows/ci.yml)
- Add requirements-dev.txt with pytest, mongomock, httpx, ruff
- Clean up typing imports and fix ruff linting across codebase

backend/database.py

@@ -1,6 +1,8 @@
-from pymongo import MongoClient, ASCENDING, DESCENDING, TEXT
-from config import MONGO_URI, DB_NAME, RETENTION_DAYS
+from contextlib import suppress
+
 import structlog
+from config import DB_NAME, MONGO_URI, RETENTION_DAYS
+from pymongo import ASCENDING, DESCENDING, TEXT, MongoClient
 
 client = MongoClient(MONGO_URI)
 db = client[DB_NAME]
@@ -29,10 +31,8 @@ def setup_indexes(max_retries: int = 5, delay: float = 2.0):
                     name="ttl_timestamp",
                 )
             else:
-                try:
+                with suppress(Exception):
                     events_collection.drop_index("ttl_timestamp")
-                except Exception:
-                    pass
             logger.info("MongoDB indexes ensured")
             return
         except Exception as exc: