Add module management system for enabling/disabling features

Stores can enable/disable feature modules from Settings. When disabled,
nav links are hidden and API routes return 403. Designed as the
foundation for future license-based gating (licensed + enabled flags).

Core modules (Accounts, Members, Users, Roles, Settings) are always on.

- module_config table with slug, name, description, licensed, enabled
- In-memory cache for fast per-request module checks
- requireModule middleware wraps route groups in main.ts
- Settings page Modules card with toggle switches
- Sidebar hides nav links for disabled modules
- Default modules seeded: inventory, pos, repairs, rentals, lessons,
  files, vault, email, reports

packages/backend/src/db/migrations/meta/_journal.json

@@ -183,6 +183,13 @@
       "when": 1774850000000,
       "tag": "0025_vault",
       "breakpoints": true
+    },
+    {
+      "idx": 26,
+      "version": "7",
+      "when": 1774860000000,
+      "tag": "0026_modules",
+      "breakpoints": true
     }
   ]
 }