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/mapping_loader.py

@@ -1,11 +1,10 @@
 from functools import lru_cache
 from pathlib import Path
-from typing import Any, Dict
+from typing import Any
 
 import yaml
 
-
-DEFAULT_MAPPING: Dict[str, Any] = {
+DEFAULT_MAPPING: dict[str, Any] = {
     "category_labels": {
         "ApplicationManagement": "Application",
         "UserManagement": "User",
@@ -38,7 +37,7 @@ DEFAULT_MAPPING: Dict[str, Any] = {
 
 
 @lru_cache(maxsize=1)
-def get_mapping() -> Dict[str, Any]:
+def get_mapping() -> dict[str, Any]:
     """
     Load mapping from mappings.yml if present; otherwise fall back to defaults.
     Users can edit mappings.yml to change labels and summary templates.