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>

package.json

@@ -19,8 +19,10 @@
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "drizzle-orm": "^0.45.1",
+    "form-data": "^4.0.1",
     "ics": "^3.8.1",
     "lucide-react": "^0.562.0",
+    "mailgun.js": "^11.1.0",
     "next": "16.1.1",
     "node-cron": "^4.2.1",
     "oauth-1.0a": "^2.2.6",
@@ -29,7 +31,6 @@
     "prom-client": "^15.1.3",
     "react": "19.2.3",
     "react-dom": "19.2.3",
-    "resend": "^6.7.0",
     "sonner": "^2.0.7",
     "tailwind-merge": "^3.4.0",
     "zod": "^4.3.5"