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>

alembic.ini

@@ -3,7 +3,7 @@
 script_location = alembic
 prepend_sys_path = .
 version_path_separator = space
-version_locations = alembic/versions app/modules/loyalty/migrations/versions
+version_locations = alembic/versions app/modules/billing/migrations/versions app/modules/cart/migrations/versions app/modules/catalog/migrations/versions app/modules/cms/migrations/versions app/modules/customers/migrations/versions app/modules/dev_tools/migrations/versions app/modules/inventory/migrations/versions app/modules/loyalty/migrations/versions app/modules/marketplace/migrations/versions app/modules/messaging/migrations/versions app/modules/orders/migrations/versions
 # This will be overridden by alembic\env.py using settings.database_url
 sqlalchemy.url =
 # for PROD: sqlalchemy.url = postgresql://username:password@localhost:5432/ecommerce_db