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/images.py

@@ -14,7 +14,7 @@ from fastapi import APIRouter, Depends, File, Form, UploadFile
 
 from app.api.deps import get_current_admin_api
 from app.services.image_service import image_service
-from models.database.user import User
+from models.schema.auth import UserContext
 from models.schema.image import (
     ImageDeleteResponse,
     ImageStorageStats,
@@ -30,7 +30,7 @@ async def upload_image(
     file: UploadFile = File(...),
     vendor_id: int = Form(...),
     product_id: int | None = Form(None),
-    current_admin: User = Depends(get_current_admin_api),
+    current_admin: UserContext = Depends(get_current_admin_api),
 ):
     """Upload and process an image.
 
@@ -67,7 +67,7 @@ async def upload_image(
 @router.delete("/{image_hash}", response_model=ImageDeleteResponse)
 async def delete_image(
     image_hash: str,
-    current_admin: User = Depends(get_current_admin_api),
+    current_admin: UserContext = Depends(get_current_admin_api),
 ):
     """Delete an image and all its variants.
 
@@ -88,7 +88,7 @@ async def delete_image(
 
 @router.get("/stats", response_model=ImageStorageStats)
 async def get_storage_stats(
-    current_admin: User = Depends(get_current_admin_api),
+    current_admin: UserContext = Depends(get_current_admin_api),
 ):
     """Get image storage statistics.