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/vendor/email_templates.py

@@ -20,7 +20,7 @@ from app.core.database import get_db
 from app.services.email_service import EmailService
 from app.services.email_template_service import EmailTemplateService
 from app.services.vendor_service import vendor_service
-from models.database.user import User
+from models.schema.auth import UserContext
 
 router = APIRouter(prefix="/email-templates")
 logger = logging.getLogger(__name__)
@@ -62,7 +62,7 @@ class TemplateTestRequest(BaseModel):
 
 @router.get("")
 def list_overridable_templates(
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -79,7 +79,7 @@ def list_overridable_templates(
 @router.get("/{code}")
 def get_template(
     code: str,
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -96,7 +96,7 @@ def get_template(
 def get_template_language(
     code: str,
     language: str,
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -114,7 +114,7 @@ def update_template_override(
     code: str,
     language: str,
     template_data: VendorTemplateUpdate,
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -144,7 +144,7 @@ def update_template_override(
 def delete_template_override(
     code: str,
     language: str,
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -168,7 +168,7 @@ def delete_template_override(
 def preview_template(
     code: str,
     preview_data: TemplatePreviewRequest,
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -201,7 +201,7 @@ def preview_template(
 def send_test_email(
     code: str,
     test_data: TemplateTestRequest,
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """