Implement token expiration warnings (P3.9)

Add email warnings for Garmin token expiration at 14-day and 7-day thresholds.
When the garmin-sync cron job runs, it now checks each user's token expiry and
sends a warning email at exactly 14 days and 7 days before expiration.

Changes:
- Add sendTokenExpirationWarning() to email.ts with differentiated subject
  lines and urgency levels for 14-day vs 7-day warnings
- Integrate warning logic into garmin-sync cron route using daysUntilExpiry()
- Track warnings sent in sync response with new warningsSent counter
- Add 20 new tests (10 for email function, 10 for sync integration)

Test count: 517 → 537

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

src/lib/email.ts

@@ -81,3 +81,37 @@ Auto-generated by PhaseFlow`;
     text: body,
   });
 }
+
+export async function sendTokenExpirationWarning(
+  to: string,
+  daysUntilExpiry: number,
+): Promise<void> {
+  const isUrgent = daysUntilExpiry <= 7;
+
+  const subject = isUrgent
+    ? `🚨 PhaseFlow: Garmin tokens expire in ${daysUntilExpiry} days - action required`
+    : `⚠️ PhaseFlow: Garmin tokens expire in ${daysUntilExpiry} days`;
+
+  const urgencyMessage = isUrgent
+    ? `⚠️ This is urgent - your Garmin data sync will stop working in ${daysUntilExpiry} days if you don't refresh your tokens.`
+    : `Your Garmin OAuth tokens will expire in ${daysUntilExpiry} days.`;
+
+  const body = `${urgencyMessage}
+
+📋 HOW TO REFRESH YOUR TOKENS:
+1. Go to Settings > Garmin in PhaseFlow
+2. Follow the instructions to reconnect your Garmin account
+3. Paste the new tokens from the bootstrap script
+
+This will ensure your training recommendations continue to use fresh Garmin data.
+
+---
+Auto-generated by PhaseFlow`;
+
+  await resend.emails.send({
+    from: EMAIL_FROM,
+    to,
+    subject,
+    text: body,
+  });
+}