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>

src/instrumentation.ts

@@ -0,0 +1,62 @@
+// ABOUTME: Next.js instrumentation file for server startup initialization.
+// ABOUTME: Schedules cron jobs for notifications and Garmin sync using node-cron.
+
+export async function register() {
+  // Only run on the server side
+  if (process.env.NEXT_RUNTIME === "nodejs") {
+    const cron = await import("node-cron");
+
+    const APP_URL = process.env.APP_URL || "http://localhost:3000";
+    const CRON_SECRET = process.env.CRON_SECRET;
+
+    // Log startup
+    console.log("[cron] Scheduler starting...");
+
+    if (!CRON_SECRET) {
+      console.warn(
+        "[cron] CRON_SECRET not set - cron jobs will fail authentication",
+      );
+    }
+
+    // Helper to call cron endpoints
+    async function triggerCronEndpoint(endpoint: string, name: string) {
+      try {
+        const response = await fetch(`${APP_URL}/api/cron/${endpoint}`, {
+          method: "POST",
+          headers: {
+            Authorization: `Bearer ${CRON_SECRET}`,
+            "Content-Type": "application/json",
+          },
+        });
+
+        if (!response.ok) {
+          const errorText = await response.text();
+          console.error(
+            `[cron] ${name} failed: ${response.status} ${errorText}`,
+          );
+        } else {
+          const result = await response.json();
+          console.log(`[cron] ${name} completed:`, result);
+        }
+      } catch (error) {
+        console.error(`[cron] ${name} error:`, error);
+      }
+    }
+
+    // Schedule notifications at the top of every hour
+    cron.default.schedule("0 * * * *", () => {
+      console.log("[cron] Triggering notifications...");
+      triggerCronEndpoint("notifications", "Notifications");
+    });
+
+    // Schedule Garmin sync at 30 minutes past every hour
+    cron.default.schedule("30 * * * *", () => {
+      console.log("[cron] Triggering Garmin sync...");
+      triggerCronEndpoint("garmin-sync", "Garmin sync");
+    });
+
+    console.log(
+      "[cron] Scheduler started - notifications at :00, Garmin sync at :30",
+    );
+  }
+}