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/tenancy/models/store_platform.py

@@ -150,17 +150,17 @@ class StorePlatform(Base, TimestampMixin):
         UniqueConstraint(
             "store_id",
             "platform_id",
-            name="uq_vendor_platform",
+            name="uq_store_platform",
         ),
         # Performance indexes
         Index(
-            "idx_vendor_platform_active",
+            "idx_store_platform_active",
             "store_id",
             "platform_id",
             "is_active",
         ),
         Index(
-            "idx_vendor_platform_primary",
+            "idx_store_platform_primary",
             "store_id",
             "is_primary",
         ),