refactor(migrations): squash 75 migrations into 12 per-module initial migrations

The old migration chain was broken (downgrade path through vendor->merchant
rename made rollbacks impossible). This squashes everything into fresh
per-module migrations with zero schema drift, verified by autogenerate.

Changes:
- Replace 75 accumulated migrations with 12 per-module initial migrations
  (core, billing, catalog, marketplace, cms, customers, orders, inventory,
  cart, messaging, loyalty, dev_tools) in a linear chain
- Fix make db-reset to use SQL DROP SCHEMA instead of alembic downgrade base
- Enable migration autodiscovery for all modules (migrations_path in definitions)
- Rewrite alembic/env.py to import all 75 model tables across 13 modules
- Fix AdminNotification import (was incorrectly from tenancy, now from messaging)
- Update squash_migrations.py to handle all module migration directories

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/marketplace/models/letzshop.py

@@ -263,9 +263,9 @@ class LetzshopStoreCache(Base, TimestampMixin):
     claimed_store = relationship("Store", foreign_keys=[claimed_by_store_id])
 
     __table_args__ = (
-        Index("idx_vendor_cache_city", "city"),
-        Index("idx_vendor_cache_claimed", "claimed_by_store_id"),
-        Index("idx_vendor_cache_active", "is_active"),
+        Index("idx_letzshop_cache_city", "city"),
+        Index("idx_letzshop_cache_claimed", "claimed_by_store_id"),
+        Index("idx_letzshop_cache_active", "is_active"),
     )
 
     def __repr__(self):