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/lib/email.ts

@@ -2,6 +2,7 @@
 // ABOUTME: Sends daily training notifications and period confirmation emails.
 import { Resend } from "resend";
 
+import { logger } from "@/lib/logger";
 import { emailSentTotal } from "@/lib/metrics";
 
 const resend = new Resend(process.env.RESEND_API_KEY);
@@ -28,7 +29,10 @@ export interface DailyEmailData {
   ketoGuidance: string;
 }
 
-export async function sendDailyEmail(data: DailyEmailData): Promise<void> {
+export async function sendDailyEmail(
+  data: DailyEmailData,
+  userId?: string,
+): Promise<void> {
   const subject = `Today's Training: ${data.decision.icon} ${data.decision.status}`;
 
   const body = `Good morning!
@@ -53,20 +57,27 @@ ${data.decision.icon} ${data.decision.reason}
 ---
 Auto-generated by PhaseFlow`;
 
-  await resend.emails.send({
-    from: EMAIL_FROM,
-    to: data.to,
-    subject,
-    text: body,
-  });
+  try {
+    await resend.emails.send({
+      from: EMAIL_FROM,
+      to: data.to,
+      subject,
+      text: body,
+    });
 
-  emailSentTotal.inc({ type: "daily" });
+    logger.info({ userId, type: "daily", recipient: data.to }, "Email sent");
+    emailSentTotal.inc({ type: "daily" });
+  } catch (err) {
+    logger.error({ userId, type: "daily", err }, "Email failed");
+    throw err;
+  }
 }
 
 export async function sendPeriodConfirmationEmail(
   to: string,
   lastPeriodDate: Date,
   cycleLength: number,
+  userId?: string,
 ): Promise<void> {
   const subject = "🔵 Period Tracking Updated";
 
@@ -78,17 +89,28 @@ Your calendar will update automatically within 24 hours.
 ---
 Auto-generated by PhaseFlow`;
 
-  await resend.emails.send({
-    from: EMAIL_FROM,
-    to,
-    subject,
-    text: body,
-  });
+  try {
+    await resend.emails.send({
+      from: EMAIL_FROM,
+      to,
+      subject,
+      text: body,
+    });
+
+    logger.info(
+      { userId, type: "period_confirmation", recipient: to },
+      "Email sent",
+    );
+  } catch (err) {
+    logger.error({ userId, type: "period_confirmation", err }, "Email failed");
+    throw err;
+  }
 }
 
 export async function sendTokenExpirationWarning(
   to: string,
   daysUntilExpiry: number,
+  userId?: string,
 ): Promise<void> {
   const isUrgent = daysUntilExpiry <= 7;
 
@@ -112,12 +134,18 @@ 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,
-  });
+  try {
+    await resend.emails.send({
+      from: EMAIL_FROM,
+      to,
+      subject,
+      text: body,
+    });
 
-  emailSentTotal.inc({ type: "warning" });
+    logger.info({ userId, type: "warning", recipient: to }, "Email sent");
+    emailSentTotal.inc({ type: "warning" });
+  } catch (err) {
+    logger.error({ userId, type: "warning", err }, "Email failed");
+    throw err;
+  }
 }