Fix CI failures: add backend/conftest.py for module resolution and run black formatting

- Add backend/conftest.py that inserts the backend directory into sys.path,
  fixing ModuleNotFoundError when pytest runs from the backend/ directory
  (as CI does with `cd backend && pytest tests/`)
- Run black formatter on all 28 backend files that needed reformatting
- All 53 tests pass with both `pytest tests/` and `python -m pytest tests/`

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/pop_puller_to_gmail/sessions/beb47db2-da25-416a-8fb0-c1452a3b22a7

backend/app/api/v1/endpoints/users.py

@@ -1,4 +1,5 @@
 """User management endpoints"""
+
 from fastapi import APIRouter, Depends, HTTPException, status
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -12,7 +13,7 @@ router = APIRouter()
 
 @router.get("/me", response_model=UserDetailResponse)
 async def get_current_user_profile(
-    current_user: User = Depends(get_current_active_user)
+    current_user: User = Depends(get_current_active_user),
 ):
     """Get current user profile"""
     return current_user
@@ -22,14 +23,14 @@ async def get_current_user_profile(
 async def update_current_user_profile(
     user_update: UserUpdate,
     current_user: User = Depends(get_current_active_user),
-    db: AsyncSession = Depends(get_db)
+    db: AsyncSession = Depends(get_db),
 ):
     """Update current user profile"""
     if user_update.email:
         current_user.email = user_update.email
     if user_update.full_name:
         current_user.full_name = user_update.full_name
-    
+
     await db.commit()
     await db.refresh(current_user)
     return current_user