Add Sentry dev probe and observability smoke test.

GET /api/debug/sentry, load .env.local for DSN, Playwright observability spec.

Co-authored-by: Cursor <cursoragent@cursor.com>

e2e/observability.spec.ts

@@ -0,0 +1,8 @@
+import { test, expect } from "@playwright/test";
+
+/** Asserts the dev Sentry probe responds; does not verify ingest in sentry.io (see README). */
+test("sentry dev probe returns ok", async ({ request }) => {
+  const res = await request.get("/api/debug/sentry");
+  expect(res.ok()).toBeTruthy();
+  await expect(res.json()).resolves.toMatchObject({ ok: true });
+});