refactor(api): introduce UserContext schema for API dependency injection

Replace direct User database model imports in API endpoints with UserContext
schema, following the architecture principle that API routes should not import
database models directly.

Changes:
- Create UserContext schema in models/schema/auth.py with from_user() factory
- Update app/api/deps.py to return UserContext from all auth dependencies
- Add _get_user_model() helper for functions needing User model access
- Update 58 API endpoint files to use UserContext instead of User
- Add noqa comments for 4 legitimate edge cases (enums, internal helpers)

Architecture validation: 0 errors (down from 61), 11 warnings remain

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/v1/admin/background_tasks.py

@@ -13,7 +13,7 @@ from sqlalchemy.orm import Session
 from app.api.deps import get_current_admin_api
 from app.core.database import get_db
 from app.services.background_tasks_service import background_tasks_service
-from models.database.user import User
+from models.schema.auth import UserContext
 
 router = APIRouter()
 
@@ -156,7 +156,7 @@ async def list_background_tasks(
     ),
     limit: int = Query(50, ge=1, le=200),
     db: Session = Depends(get_db),
-    current_user: User = Depends(get_current_admin_api),
+    current_user: UserContext = Depends(get_current_admin_api),
 ):
     """
     List all background tasks across the system
@@ -198,7 +198,7 @@ async def list_background_tasks(
 @router.get("/tasks/stats", response_model=BackgroundTasksStatsResponse)
 async def get_background_tasks_stats(
     db: Session = Depends(get_db),
-    current_user: User = Depends(get_current_admin_api),
+    current_user: UserContext = Depends(get_current_admin_api),
 ):
     """
     Get statistics for background tasks
@@ -236,7 +236,7 @@ async def get_background_tasks_stats(
 @router.get("/tasks/running", response_model=list[BackgroundTaskResponse])
 async def list_running_tasks(
     db: Session = Depends(get_db),
-    current_user: User = Depends(get_current_admin_api),
+    current_user: UserContext = Depends(get_current_admin_api),
 ):
     """
     List currently running background tasks