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_stores.py

@@ -87,6 +87,8 @@ def get_all_stores_admin(
     is_active: bool | None = Query(None),
     is_verified: bool | None = Query(None),
     merchant_id: int | None = Query(None, description="Filter by merchant ID"),
+    include_deleted: bool = Query(False, description="Include soft-deleted stores"),
+    only_deleted: bool = Query(False, description="Show only soft-deleted stores (trash view)"),
     db: Session = Depends(get_db),
     current_admin: UserContext = Depends(get_current_admin_api),
 ):
@@ -99,6 +101,8 @@ def get_all_stores_admin(
         is_active=is_active,
         is_verified=is_verified,
         merchant_id=merchant_id,
+        include_deleted=include_deleted,
+        only_deleted=only_deleted,
     )
     return StoreListResponse(stores=stores, total=total, skip=skip, limit=limit)
 
@@ -309,3 +313,24 @@ def delete_store(
     message = admin_service.delete_store(db, store.id)
     db.commit()
     return {"message": message}
+
+
+@admin_stores_router.put("/{store_id}/restore")
+def restore_store(
+    store_id: int,
+    db: Session = Depends(get_db),
+    current_admin: UserContext = Depends(get_current_admin_api),
+):
+    """
+    Restore a soft-deleted store (Admin only).
+
+    This only restores the store record itself.
+    Child records (products, customers, etc.) must be restored separately.
+    """
+    from app.core.soft_delete import restore
+    from app.modules.tenancy.models import Store
+
+    restored = restore(db, Store, store_id, restored_by_id=current_admin.id)
+    db.commit()
+    logger.info(f"Store {store_id} restored by admin {current_admin.username}")
+    return {"message": f"Store '{restored.name}' restored successfully", "store_id": store_id}