Switch email provider from Resend to Mailgun and add cron scheduler

- Replace resend package with mailgun.js and form-data
- Update email.ts to use Mailgun API with lazy client initialization
- Add instrumentation.ts to schedule cron jobs (notifications :00, Garmin :30)
- Update tests for Mailgun mock structure
- Update .env.example with MAILGUN_API_KEY and MAILGUN_DOMAIN

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.env.example

@@ -11,8 +11,9 @@ NODE_ENV=development
 POCKETBASE_URL=http://localhost:8090
 NEXT_PUBLIC_POCKETBASE_URL=http://localhost:8090
 
-# Email (Resend)
-RESEND_API_KEY=re_xxxxxxxxxxxx
+# Email (Mailgun)
+MAILGUN_API_KEY=key-xxxxxxxxxxxx
+MAILGUN_DOMAIN=yourdomain.com
 EMAIL_FROM=phaseflow@yourdomain.com
 
 # Encryption (for Garmin tokens)