refactor(arch): move auth schemas to tenancy module and add cross-module service methods

Move all auth schemas (UserContext, UserLogin, LoginResponse, etc.) from
legacy models/schema/auth.py to app/modules/tenancy/schemas/auth.py per
MOD-019. Update 84 import sites across 14 modules. Legacy file now
re-exports for backwards compatibility.

Add missing tenancy service methods for cross-module consumers:
- merchant_service.get_merchant_by_owner_id()
- merchant_service.get_merchant_count_for_owner()
- admin_service.get_user_by_id() (public, was private-only)
- platform_service.get_active_store_count()

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/cms/routes/api/admin_images.py

@@ -15,7 +15,7 @@ from app.api.deps import get_current_admin_api
 from app.core.database import get_db
 from app.modules.cms.schemas.image import ImageStorageStats
 from app.modules.cms.services.media_service import media_service
-from models.schema.auth import UserContext
+from app.modules.tenancy.schemas.auth import UserContext
 
 admin_images_router = APIRouter(prefix="/images")
 logger = logging.getLogger(__name__)

app/modules/cms/routes/api/admin_media.py

@@ -19,7 +19,7 @@ from app.modules.cms.schemas.media import (
     MediaUploadResponse,
 )
 from app.modules.cms.services.media_service import media_service
-from models.schema.auth import UserContext
+from app.modules.tenancy.schemas.auth import UserContext
 
 admin_media_router = APIRouter(prefix="/media")
 logger = logging.getLogger(__name__)

app/modules/cms/routes/api/admin_store_themes.py

@@ -26,7 +26,7 @@ from app.modules.cms.schemas.store_theme import (
     ThemePresetResponse,
 )
 from app.modules.cms.services.store_theme_service import store_theme_service
-from models.schema.auth import UserContext
+from app.modules.tenancy.schemas.auth import UserContext
 
 admin_store_themes_router = APIRouter(prefix="/store-themes")
 logger = logging.getLogger(__name__)

app/modules/cms/routes/api/store_media.py

@@ -27,7 +27,7 @@ from app.modules.cms.schemas.media import (
     UploadedFileInfo,
 )
 from app.modules.cms.services.media_service import media_service
-from models.schema.auth import UserContext
+from app.modules.tenancy.schemas.auth import UserContext
 
 store_media_router = APIRouter(prefix="/media")
 logger = logging.getLogger(__name__)