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/db/index.ts

@@ -3,3 +3,4 @@ export * from './schema/users.js'
 export * from './schema/accounts.js'
 export * from './schema/inventory.js'
 export * from './schema/pos.js'
+export * from './schema/settings.js'