Fix all 33 ruff linting errors causing CI failures

- Remove unused imports (F401) across 17 files
- Fix f-strings without placeholders (F541) in 3 files
- Add noqa: E712 to SQLAlchemy == True comparisons (valid ORM pattern)
- Preserve alembic side-effect import with noqa: F401

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
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backend/app/api/v1/endpoints/subscriptions.py

@@ -1,7 +1,7 @@
 """Subscription and payment endpoints"""
 
 from typing import List
-from fastapi import APIRouter, Depends, HTTPException, status
+from fastapi import APIRouter, Depends
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy import select
 
@@ -17,7 +17,7 @@ router = APIRouter()
 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)
+        select(SubscriptionPlan).where(SubscriptionPlan.is_active == True)  # noqa: E712
     )
     return result.scalars().all()