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>

Makefile

@@ -162,8 +162,8 @@ db-setup: migrate-up init-prod seed-demo
 
 db-reset:
 	@echo "⚠️  WARNING: This will DELETE ALL existing data!"
-	@echo "Rolling back all migrations..."
-	$(PYTHON) -m alembic downgrade base || true
+	@echo "Dropping and recreating public schema..."
+	$(PYTHON) -c "from app.core.config import settings; from sqlalchemy import create_engine, text; e=create_engine(settings.database_url); c=e.connect(); c.execute(text('DROP SCHEMA public CASCADE')); c.execute(text('CREATE SCHEMA public')); c.commit(); c.close()"
 	@echo "Applying all migrations..."
 	$(PYTHON) -m alembic upgrade head
 	@echo "Initializing production data..."