feat: add app settings table, encryption utility, and generic email service

- app_settings table with encrypted field support (AES-256-GCM, key from ENCRYPTION_KEY env)
- SettingsService for transparent encrypt/decrypt on get/set
- EmailService factory with Resend and SendGrid providers (SMTP stub) — provider config lives in app_settings
- Seeds initial admin user and email settings from env vars on first startup if not already present
- Migration 0039_app_settings.sql

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/backend/src/plugins/database.ts

@@ -6,8 +6,9 @@ import * as userSchema from '../db/schema/users.js'
 import * as accountSchema from '../db/schema/accounts.js'
 import * as inventorySchema from '../db/schema/inventory.js'
 import * as posSchema from '../db/schema/pos.js'
+import * as settingsSchema from '../db/schema/settings.js'
 
-const schema = { ...storeSchema, ...userSchema, ...accountSchema, ...inventorySchema, ...posSchema }
+const schema = { ...storeSchema, ...userSchema, ...accountSchema, ...inventorySchema, ...posSchema, ...settingsSchema }
 
 declare module 'fastify' {
   interface FastifyInstance {