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/loyalty/services/program_service.py

@@ -568,19 +568,23 @@ class ProgramService:
         return program
 
     def delete_program(self, db: Session, program_id: int) -> None:
-        """Delete a loyalty program and all associated data."""
+        """Soft-delete a loyalty program and associated cards."""
+        from app.core.soft_delete import soft_delete_cascade
+
         program = self.require_program(db, program_id)
         merchant_id = program.merchant_id
 
-        # Also delete merchant settings
+        # Hard delete merchant settings (config data, not business records)
         db.query(MerchantLoyaltySettings).filter(
             MerchantLoyaltySettings.merchant_id == merchant_id
         ).delete()
 
-        db.delete(program)
+        soft_delete_cascade(db, program, deleted_by_id=None, cascade_rels=[
+            ("cards", []),
+        ])
         db.commit()
 
-        logger.info(f"Deleted loyalty program {program_id} for merchant {merchant_id}")
+        logger.info(f"Soft-deleted loyalty program {program_id} for merchant {merchant_id}")
 
     # =========================================================================
     # Merchant Settings