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/customers/routes/api/admin.py

@@ -18,7 +18,7 @@ from app.modules.customers.schemas import (
 )
 from app.modules.customers.services import admin_customer_service
 from app.modules.enums import FrontendType
-from models.schema.auth import UserContext
+from app.modules.tenancy.schemas.auth import UserContext
 
 # Create module-aware router
 admin_router = APIRouter(

app/modules/customers/routes/api/store.py

@@ -22,7 +22,7 @@ from app.modules.customers.schemas import (
 )
 from app.modules.customers.services import customer_service
 from app.modules.enums import FrontendType
-from models.schema.auth import UserContext
+from app.modules.tenancy.schemas.auth import UserContext
 
 # Create module-aware router
 store_router = APIRouter(
@@ -150,4 +150,3 @@ def toggle_customer_status(
 
     status = "activated" if customer.is_active else "deactivated"
     return CustomerMessageResponse(message=f"Customer {status} successfully")
-

app/modules/customers/routes/api/storefront.py

@@ -47,7 +47,7 @@ from app.modules.messaging.services.email_service import (
     EmailService,  # MOD-004 - Core email service
 )
 from app.modules.tenancy.exceptions import StoreNotFoundException
-from models.schema.auth import (
+from app.modules.tenancy.schemas.auth import (
     LogoutResponse,
     PasswordResetRequestResponse,
     PasswordResetResponse,