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/merchant.py

@@ -10,10 +10,10 @@ from sqlalchemy import Boolean, Column, ForeignKey, Integer, String, Text
 from sqlalchemy.orm import relationship
 
 from app.core.database import Base
-from models.database.base import TimestampMixin
+from models.database.base import SoftDeleteMixin, TimestampMixin
 
 
-class Merchant(Base, TimestampMixin):
+class Merchant(Base, TimestampMixin, SoftDeleteMixin):
     """
     Represents a merchant (business entity) in the system.
 
@@ -74,7 +74,7 @@ class Merchant(Base, TimestampMixin):
     # ========================================================================
     # Relationships
     # ========================================================================
-    owner = relationship("User", back_populates="owned_merchants")
+    owner = relationship("User", foreign_keys="[Merchant.owner_user_id]", back_populates="owned_merchants")
     """The user who owns this merchant."""
 
     stores = relationship(