Add email sent/failed structured logging

Implement email logging per observability spec:
- Add structured logging for email sent (info level) and failed (error level)
- Include userId, type, and recipient fields in log events
- Add userId parameter to email functions (sendDailyEmail, sendPeriodConfirmationEmail, sendTokenExpirationWarning)
- Update cron routes (notifications, garmin-sync) to pass userId

6 new tests added to email.test.ts (now 30 tests total)

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

src/app/api/cron/garmin-sync/route.ts

@@ -82,7 +82,7 @@ export async function POST(request: Request) {
       const daysRemaining = daysUntilExpiry(tokens);
       if (daysRemaining === 14 || daysRemaining === 7) {
         try {
-          await sendTokenExpirationWarning(user.email, daysRemaining);
+          await sendTokenExpirationWarning(user.email, daysRemaining, user.id);
           result.warningsSent++;
         } catch {
           // Continue processing even if warning email fails