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/models/store.py

@@ -14,6 +14,7 @@ from sqlalchemy import (
     Column,
     DateTime,
     ForeignKey,
+    Index,
     Integer,
     String,
     Text,
@@ -24,13 +25,17 @@ from app.core.config import settings
 
 # Import Base from the central database module instead of creating a new one
 from app.core.database import Base
-from models.database.base import TimestampMixin
+from models.database.base import SoftDeleteMixin, TimestampMixin
 
 
-class Store(Base, TimestampMixin):
+class Store(Base, TimestampMixin, SoftDeleteMixin):
     """Represents a store in the system."""
 
     __tablename__ = "stores"  # Name of the table in the database
+    __table_args__ = (
+        Index("uq_stores_store_code_active", "store_code", unique=True, postgresql_where="deleted_at IS NULL"),
+        Index("uq_stores_subdomain_active", "subdomain", unique=True, postgresql_where="deleted_at IS NULL"),
+    )
 
     id = Column(
         Integer, primary_key=True, index=True
@@ -42,11 +47,11 @@ class Store(Base, TimestampMixin):
     )  # Foreign key to the parent merchant
 
     store_code = Column(
-        String, unique=True, index=True, nullable=False
-    )  # Unique, indexed, non-nullable store code column
+        String, index=True, nullable=False
+    )  # Indexed, non-nullable store code column (unique among non-deleted)
     subdomain = Column(
-        String(100), unique=True, nullable=False, index=True
-    )  # Unique, non-nullable subdomain column with indexing
+        String(100), nullable=False, index=True
+    )  # Non-nullable subdomain column (unique among non-deleted)
     name = Column(
         String, nullable=False
     )  # Non-nullable name column for the store (brand name)
@@ -418,7 +423,7 @@ class Store(Base, TimestampMixin):
         }
 
 
-class StoreUser(Base, TimestampMixin):
+class StoreUser(Base, TimestampMixin, SoftDeleteMixin):
     """
     Represents a user's team membership in a store.