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/subscriptions.py

@@ -1,4 +1,5 @@
 """Subscription and payment endpoints"""
+
 from typing import List
 from fastapi import APIRouter, Depends, HTTPException, status
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -13,9 +14,7 @@ router = APIRouter()
 
 
 @router.get("/plans", response_model=List[SubscriptionPlanResponse])
-async def list_subscription_plans(
-    db: AsyncSession = Depends(get_db)
-):
+async def list_subscription_plans(db: AsyncSession = Depends(get_db)):
     """List all available subscription plans"""
     result = await db.execute(
         select(SubscriptionPlan).where(SubscriptionPlan.is_active == True)
@@ -25,11 +24,11 @@ async def list_subscription_plans(
 
 @router.get("/current")
 async def get_current_subscription(
-    current_user: User = Depends(get_current_active_user)
+    current_user: User = Depends(get_current_active_user),
 ):
     """Get current user's subscription details"""
     return {
         "tier": current_user.subscription_tier,
         "status": current_user.subscription_status,
-        "expires_at": current_user.subscription_expires_at
+        "expires_at": current_user.subscription_expires_at,
     }