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/0026_modules.sql

@@ -0,0 +1,24 @@
+-- Module configuration table for enabling/disabling feature modules
+
+CREATE TABLE module_config (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  slug VARCHAR(50) NOT NULL UNIQUE,
+  name VARCHAR(100) NOT NULL,
+  description TEXT,
+  licensed BOOLEAN NOT NULL DEFAULT true,
+  enabled BOOLEAN NOT NULL DEFAULT false,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+  updated_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+-- Seed default modules
+INSERT INTO module_config (slug, name, description, licensed, enabled) VALUES
+  ('inventory', 'Inventory', 'Product catalog, stock tracking, and unit management', true, true),
+  ('pos', 'Point of Sale', 'Sales transactions, cash drawer, and receipts', true, true),
+  ('repairs', 'Repairs', 'Repair ticket management, batches, and service templates', true, true),
+  ('rentals', 'Rentals', 'Instrument rental agreements and billing', true, false),
+  ('lessons', 'Lessons', 'Lesson scheduling, instructor management, and billing', true, false),
+  ('files', 'Files', 'Shared file storage with folder organization', true, true),
+  ('vault', 'Vault', 'Encrypted password and secret manager', true, true),
+  ('email', 'Email', 'Email campaigns, templates, and sending', true, false),
+  ('reports', 'Reports', 'Business reports and data export', true, true);