Add test endpoint for verifying email configuration

POST /api/test/email with {"to": "email@example.com"} to send a test email.
Protected by CRON_SECRET.

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

src/app/api/test/email/route.ts

@@ -0,0 +1,59 @@
+// ABOUTME: Test endpoint for verifying email configuration.
+// ABOUTME: Sends a test email to verify Mailgun integration works.
+import { NextResponse } from "next/server";
+import type { DailyEmailData } from "@/lib/email";
+import { sendDailyEmail } from "@/lib/email";
+
+export async function POST(request: Request) {
+  // Verify cron secret (reuse same auth as cron endpoints)
+  const authHeader = request.headers.get("authorization");
+  const expectedSecret = process.env.CRON_SECRET;
+
+  if (!expectedSecret || authHeader !== `Bearer ${expectedSecret}`) {
+    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  }
+
+  const body = await request.json().catch(() => ({}));
+  const to = body.to as string;
+
+  if (!to) {
+    return NextResponse.json(
+      { error: "Missing 'to' email address in request body" },
+      { status: 400 },
+    );
+  }
+
+  const testData: DailyEmailData = {
+    to,
+    cycleDay: 15,
+    phase: "OVULATION",
+    decision: {
+      status: "TRAIN",
+      reason: "This is a test email to verify Mailgun configuration works!",
+      icon: "🧪",
+    },
+    bodyBatteryCurrent: 85,
+    bodyBatteryYesterdayLow: 45,
+    hrvStatus: "Balanced",
+    weekIntensity: 60,
+    phaseLimit: 80,
+    remainingMinutes: 20,
+    seeds: "Sesame (1-2 tbsp) + Sunflower (1-2 tbsp)",
+    carbRange: "100-150g",
+    ketoGuidance: "No - exit keto, need carbs for ovulation",
+  };
+
+  try {
+    await sendDailyEmail(testData, "test-user");
+    return NextResponse.json({
+      success: true,
+      message: `Test email sent to ${to}`,
+    });
+  } catch (error) {
+    const message = error instanceof Error ? error.message : "Unknown error";
+    return NextResponse.json(
+      { success: false, error: message },
+      { status: 500 },
+    );
+  }
+}