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/tenancy/schemas/__init__.py

@@ -5,7 +5,7 @@ Tenancy module Pydantic schemas.
 Request/response schemas for platform, merchant, store, admin user, and team management.
 """
 
-# Merchant schemas
+# Auth schemas
 # Admin schemas
 from app.modules.tenancy.schemas.admin import (
     AdminAuditLogFilters,
@@ -49,6 +49,27 @@ from app.modules.tenancy.schemas.admin import (
     RowsPerPageUpdateResponse,
     SystemHealthResponse,
 )
+from app.modules.tenancy.schemas.auth import (
+    LoginResponse,
+    LogoutResponse,
+    OwnedMerchantSummary,
+    PasswordResetRequestResponse,
+    PasswordResetResponse,
+    PlatformSelectResponse,
+    StoreMembershipSummary,
+    StoreUserResponse,
+    UserContext,
+    UserCreate,
+    UserDeleteResponse,
+    UserDetailResponse,
+    UserListResponse,
+    UserLogin,
+    UserResponse,
+    UserSearchItem,
+    UserSearchResponse,
+    UserStatusToggleResponse,
+    UserUpdate,
+)
 from app.modules.tenancy.schemas.merchant import (
     MerchantBase,
     MerchantCreate,
@@ -112,6 +133,26 @@ from app.modules.tenancy.schemas.team import (
 )
 
 __all__ = [
+    # Auth
+    "LoginResponse",
+    "LogoutResponse",
+    "OwnedMerchantSummary",
+    "PasswordResetRequestResponse",
+    "PasswordResetResponse",
+    "PlatformSelectResponse",
+    "StoreMembershipSummary",
+    "StoreUserResponse",
+    "UserContext",
+    "UserCreate",
+    "UserDeleteResponse",
+    "UserDetailResponse",
+    "UserListResponse",
+    "UserLogin",
+    "UserResponse",
+    "UserSearchItem",
+    "UserSearchResponse",
+    "UserStatusToggleResponse",
+    "UserUpdate",
     # Merchant
     "MerchantBase",
     "MerchantCreate",