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/messaging/models/store_email_settings.py

@@ -136,7 +136,7 @@ class StoreEmailSettings(Base, TimestampMixin):
     # Indexes
     # =========================================================================
     __table_args__ = (
-        Index("idx_vendor_email_settings_configured", "store_id", "is_configured"),
+        Index("idx_store_email_settings_configured", "store_id", "is_configured"),
     )
 
     def __repr__(self) -> str:

app/modules/messaging/models/store_email_template.py

@@ -75,7 +75,7 @@ class StoreEmailTemplate(Base, TimestampMixin):
             "store_id",
             "template_code",
             "language",
-            name="uq_vendor_email_template_code_language",
+            name="uq_store_email_template_code_language",
         ),
         {"sqlite_autoincrement": True},
     )