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/dev_tools/definition.py

@@ -85,6 +85,7 @@ dev_tools_module = ModuleDefinition(
     schemas_path="app.modules.dev_tools.schemas",
     exceptions_path="app.modules.dev_tools.exceptions",
     tasks_path="app.modules.dev_tools.tasks",
+    migrations_path="migrations",
     # Note: Code quality and test tasks are on-demand, not scheduled.
     # If scheduled scans are desired, add ScheduledTask entries here.
 )