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/routes/api/admin_users.py

@@ -143,6 +143,8 @@ def list_admin_users(
     skip: int = Query(0, ge=0),
     limit: int = Query(100, ge=1, le=500),
     include_super_admins: bool = Query(True),
+    include_deleted: bool = Query(False, description="Include soft-deleted users"),
+    only_deleted: bool = Query(False, description="Show only soft-deleted users (trash view)"),
     db: Session = Depends(get_db),
     current_admin: UserContext = Depends(get_current_super_admin),
 ):
@@ -156,6 +158,8 @@ def list_admin_users(
         skip=skip,
         limit=limit,
         include_super_admins=include_super_admins,
+        include_deleted=include_deleted,
+        only_deleted=only_deleted,
     )
 
     admin_responses = [_build_admin_response(admin) for admin in admins]
@@ -395,3 +399,26 @@ def delete_admin_user(
         "message": "Admin user deleted successfully",
         "user_id": user_id,
     }
+
+
+@admin_users_router.put("/{user_id}/restore")
+def restore_admin_user(
+    user_id: int = Path(...),
+    db: Session = Depends(get_db),
+    current_admin: UserContext = Depends(get_current_super_admin_api),
+):
+    """
+    Restore a soft-deleted admin user.
+
+    Super admin only.
+    """
+    from app.core.soft_delete import restore
+    from app.modules.tenancy.models import User
+
+    restored = restore(db, User, user_id, restored_by_id=current_admin.id)
+    db.commit()
+
+    return {
+        "message": f"User '{restored.username}' restored successfully",
+        "user_id": user_id,
+    }