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/merchant_service.py

@@ -148,6 +148,8 @@ class MerchantService:
         search: str | None = None,
         is_active: bool | None = None,
         is_verified: bool | None = None,
+        include_deleted: bool = False,
+        only_deleted: bool = False,
     ) -> tuple[list[Merchant], int]:
         """
         Get paginated list of merchants with optional filters.
@@ -159,15 +161,25 @@ class MerchantService:
             search: Search term for merchant name
             is_active: Filter by active status
             is_verified: Filter by verified status
+            include_deleted: Include soft-deleted merchants
+            only_deleted: Show only soft-deleted merchants (trash view)
 
         Returns:
             Tuple of (merchants list, total count)
         """
+        exec_opts = {}
+        if include_deleted or only_deleted:
+            exec_opts["include_deleted"] = True
+
         query = select(Merchant).options(
             joinedload(Merchant.stores),
             joinedload(Merchant.owner),
         )
 
+        # Soft-delete filter
+        if only_deleted:
+            query = query.where(Merchant.deleted_at.isnot(None))
+
         # Apply filters
         if search:
             query = query.where(Merchant.name.ilike(f"%{search}%"))
@@ -178,13 +190,13 @@ class MerchantService:
 
         # Get total count
         count_query = select(func.count()).select_from(query.subquery())
-        total = db.execute(count_query).scalar()
+        total = db.execute(count_query, execution_options=exec_opts).scalar()
 
         # Apply pagination and order
         query = query.order_by(Merchant.name).offset(skip).limit(limit)
 
         # Use unique() when using joinedload with collections to avoid duplicate rows
-        merchants = list(db.execute(query).scalars().unique().all())
+        merchants = list(db.execute(query, execution_options=exec_opts).scalars().unique().all())
 
         return merchants, total
 
@@ -228,11 +240,19 @@ class MerchantService:
         Raises:
             MerchantNotFoundException: If merchant not found
         """
+        from app.core.soft_delete import soft_delete_cascade
+
         merchant = self.get_merchant_by_id(db, merchant_id)
 
-        # Due to cascade="all, delete-orphan", associated stores will be deleted
-        db.delete(merchant)
-        db.flush()
+        MERCHANT_CASCADE = [
+            ("stores", [
+                ("products", []),
+                ("customers", []),
+                ("orders", []),
+                ("store_users", []),
+            ]),
+        ]
+        soft_delete_cascade(db, merchant, deleted_by_id=None, cascade_rels=MERCHANT_CASCADE)
         logger.info(f"Deleted merchant ID {merchant_id} and associated stores")
 
     def toggle_verification(