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/vendor_themes.py

@@ -19,7 +19,7 @@ from sqlalchemy.orm import Session
 
 from app.api.deps import get_current_admin_api, get_db
 from app.services.vendor_theme_service import vendor_theme_service
-from models.database.user import User
+from models.schema.auth import UserContext
 from models.schema.vendor_theme import (
     ThemeDeleteResponse,
     ThemePresetListResponse,
@@ -38,7 +38,7 @@ logger = logging.getLogger(__name__)
 
 
 @router.get("/presets", response_model=ThemePresetListResponse)
-async def get_theme_presets(current_admin: User = Depends(get_current_admin_api)):
+async def get_theme_presets(current_admin: UserContext = Depends(get_current_admin_api)):
     """
     Get all available theme presets with preview information.
 
@@ -65,7 +65,7 @@ async def get_theme_presets(current_admin: User = Depends(get_current_admin_api)
 async def get_vendor_theme(
     vendor_code: str = Path(..., description="Vendor code"),
     db: Session = Depends(get_db),
-    current_admin: User = Depends(get_current_admin_api),
+    current_admin: UserContext = Depends(get_current_admin_api),
 ):
     """
     Get theme configuration for a vendor.
@@ -101,7 +101,7 @@ async def update_vendor_theme(
     vendor_code: str = Path(..., description="Vendor code"),
     theme_data: VendorThemeUpdate = None,
     db: Session = Depends(get_db),
-    current_admin: User = Depends(get_current_admin_api),
+    current_admin: UserContext = Depends(get_current_admin_api),
 ):
     """
     Update or create theme for a vendor.
@@ -150,7 +150,7 @@ async def apply_theme_preset(
     vendor_code: str = Path(..., description="Vendor code"),
     preset_name: str = Path(..., description="Preset name"),
     db: Session = Depends(get_db),
-    current_admin: User = Depends(get_current_admin_api),
+    current_admin: UserContext = Depends(get_current_admin_api),
 ):
     """
     Apply a theme preset to a vendor.
@@ -203,7 +203,7 @@ async def apply_theme_preset(
 async def delete_vendor_theme(
     vendor_code: str = Path(..., description="Vendor code"),
     db: Session = Depends(get_db),
-    current_admin: User = Depends(get_current_admin_api),
+    current_admin: UserContext = Depends(get_current_admin_api),
 ):
     """
     Delete custom theme for a vendor.