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.test.ts

@@ -424,6 +424,7 @@ describe("POST /api/cron/garmin-sync", () => {
       expect(mockSendTokenExpirationWarning).toHaveBeenCalledWith(
         "user@example.com",
         14,
+        "user123",
       );
       const body = await response.json();
       expect(body.warningsSent).toBe(1);
@@ -438,6 +439,7 @@ describe("POST /api/cron/garmin-sync", () => {
       expect(mockSendTokenExpirationWarning).toHaveBeenCalledWith(
         "user@example.com",
         7,
+        "user123",
       );
       const body = await response.json();
       expect(body.warningsSent).toBe(1);
@@ -493,10 +495,12 @@ describe("POST /api/cron/garmin-sync", () => {
       expect(mockSendTokenExpirationWarning).toHaveBeenCalledWith(
         "user1@example.com",
         14,
+        "user1",
       );
       expect(mockSendTokenExpirationWarning).toHaveBeenCalledWith(
         "user2@example.com",
         7,
+        "user2",
       );
       const body = await response.json();
       expect(body.warningsSent).toBe(2);