feat(arch): implement soft delete for business-critical models

Adds SoftDeleteMixin (deleted_at + deleted_by_id) with automatic query
filtering via do_orm_execute event. Soft-deleted records are invisible
by default; bypass with execution_options={"include_deleted": True}.

Models: User, Merchant, Store, StoreUser, Customer, Order, Product,
LoyaltyProgram, LoyaltyCard.

Infrastructure:
- SoftDeleteMixin in models/database/base.py
- Auto query filter registered on SessionLocal and test sessions
- soft_delete(), restore(), soft_delete_cascade() in app/core/soft_delete.py
- Alembic migration adding columns to 9 tables
- Partial unique indexes on users.email/username, stores.store_code/subdomain

Service changes:
- admin_service: delete_user, delete_store → soft_delete/soft_delete_cascade
- merchant_service: delete_merchant → soft_delete_cascade (stores→children)
- store_team_service: remove_team_member → soft_delete (fixes is_active bug)
- product_service: delete_product → soft_delete
- program_service: delete_program → soft_delete_cascade

Admin API:
- include_deleted/only_deleted query params on admin list endpoints
- PUT restore endpoints for users, merchants, stores

Tests: 9 unit tests for soft-delete infrastructure.
Docs: docs/backend/soft-delete.md + follow-up proposals.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/tenancy/services/admin_platform_service.py

@@ -443,6 +443,8 @@ class AdminPlatformService:
         include_super_admins: bool = True,
         is_active: bool | None = None,
         search: str | None = None,
+        include_deleted: bool = False,
+        only_deleted: bool = False,
     ) -> tuple[list[User], int]:
         """
         List all admin users with optional filtering.
@@ -454,6 +456,8 @@ class AdminPlatformService:
             include_super_admins: Whether to include super admins
             is_active: Filter by active status
             search: Search term for username/email/name
+            include_deleted: Include soft-deleted users
+            only_deleted: Show only soft-deleted users
 
         Returns:
             Tuple of (list of User objects, total count)
@@ -462,6 +466,12 @@ class AdminPlatformService:
             User.role.in_(["super_admin", "platform_admin"])
         )
 
+        # Soft-delete visibility
+        if include_deleted or only_deleted:
+            query = query.execution_options(include_deleted=True)
+        if only_deleted:
+            query = query.filter(User.deleted_at.isnot(None))
+
         if not include_super_admins:
             query = query.filter(User.role == "platform_admin")