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

pyproject.toml

@@ -0,0 +1,14 @@
+[tool.ruff]
+target-version = "py311"
+line-length = 120
+
+[tool.ruff.lint]
+select = ["E", "F", "I", "N", "W", "UP", "B", "C4", "SIM"]
+ignore = ["E501"]
+
+[tool.ruff.lint.pydocstyle]
+convention = "google"
+
+[tool.pytest.ini_options]
+testpaths = ["backend/tests"]
+pythonpath = ["backend"]