feat: admin settings page - Stripe/Twilio/WhatsApp config UI
- Add AdminSettings page with 4 tabs: Integrations, WhatsApp, Notifications, General
- Add tRPC admin endpoints: listConfig, setConfig, deleteConfig, testStripeConnection, testSmsConnection
- Add clinicSettings.toggleSms endpoint for per-clinic SMS toggle
- Add app_config table schema + DB helpers (listAllConfig, setConfigValue, deleteConfigValue)
- Stripe and SMS services now read config from DB first, then env vars fallback
- Add Settings nav item in sidebar (admin only)
- Add /admin/settings route in App.tsx
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