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/main.py

@@ -1,17 +1,18 @@
 import asyncio
 import logging
+from contextlib import suppress
 from pathlib import Path
 
 import structlog
-from fastapi import FastAPI, HTTPException
-from fastapi.staticfiles import StaticFiles
-from fastapi.middleware.cors import CORSMiddleware
-
+from config import CORS_ORIGINS, ENABLE_PERIODIC_FETCH, FETCH_INTERVAL_MINUTES
 from database import setup_indexes
-from routes.fetch import router as fetch_router, run_fetch
-from routes.events import router as events_router
+from fastapi import FastAPI, HTTPException
+from fastapi.middleware.cors import CORSMiddleware
+from fastapi.staticfiles import StaticFiles
 from routes.config import router as config_router
-from config import ENABLE_PERIODIC_FETCH, FETCH_INTERVAL_MINUTES, CORS_ORIGINS
+from routes.events import router as events_router
+from routes.fetch import router as fetch_router
+from routes.fetch import run_fetch
 
 
 def configure_logging():
@@ -90,7 +91,5 @@ async def stop_periodic_fetch():
     task = getattr(app.state, "fetch_task", None)
     if task:
         task.cancel()
-        try:
+        with suppress(Exception):
             await task
-        except Exception:
-            pass