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/models/api.py

@@ -0,0 +1,41 @@
+from pydantic import BaseModel, ConfigDict
+
+
+class EventItem(BaseModel):
+    id: str | None = None
+    timestamp: str | None = None
+    service: str | None = None
+    operation: str | None = None
+    result: str | None = None
+    actor_display: str | None = None
+    target_displays: list[str] | None = None
+    display_summary: str | None = None
+    display_category: str | None = None
+    dedupe_key: str | None = None
+    actor: dict | None = None
+    targets: list[dict] | None = None
+    raw: dict | None = None
+    raw_text: str | None = None
+
+    model_config = ConfigDict(extra="allow")
+
+
+class PaginatedEventResponse(BaseModel):
+    items: list[dict]
+    total: int
+    page: int
+    page_size: int
+
+
+class FilterOptionsResponse(BaseModel):
+    services: list[str]
+    operations: list[str]
+    results: list[str]
+    actors: list[str]
+    actor_upns: list[str]
+    devices: list[str]
+
+
+class FetchAuditLogsResponse(BaseModel):
+    stored_events: int
+    errors: list[str]

backend/models/event_model.py

@@ -2,7 +2,6 @@ import json
 
 from mapping_loader import get_mapping
 
-
 CATEGORY_LABELS = {
     "ApplicationManagement": "Application",
     "UserManagement": "User",