feat: add /readyz readiness probe with database check (#113)

* feat: add /readyz readiness probe with database check

* fix: set no-store on /readyz responses and add 503-path unit test

Addresses review nits on #113:
- Mark both the 200/ready and 503/unavailable responses Cache-Control: no-store
  so an intermediary cache can't serve a stale readiness verdict.
- Add tests/unit/readyz-endpoint.test.ts covering both branches by stubbing
  db.execute, locking in the 503/down shape that the e2e test can't reach.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: dobby-yivi-agent[bot] <275734547+dobby-yivi-agent[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: 3 people

src/routes/readyz/+server.ts

@@ -0,0 +1,27 @@
+import { json } from '@sveltejs/kit';
+import { sql } from 'drizzle-orm';
+import { db } from '$lib/server/db';
+import type { RequestHandler } from './$types';
+
+// Readiness probe: reports 200 only when the app can actually serve traffic
+// (i.e. the database is reachable). This is distinct from /health, which is a
+// pure liveness check. Orchestrator readiness gates should point here; the
+// container liveness HEALTHCHECK stays on /health.
+// A readiness verdict must never be cached: the endpoint flips between
+// 200/ready and 503/unavailable, and a stale cached response could mis-gate
+// traffic. Mark every response no-store so no intermediary reuses it.
+const NO_STORE = { 'Cache-Control': 'no-store' };
+
+export const GET: RequestHandler = async () => {
+	try {
+		await db.execute(sql`select 1`);
+	} catch (err) {
+		console.error('[readyz] database check failed', err);
+		return json(
+			{ status: 'unavailable', checks: { database: 'down' } },
+			{ status: 503, headers: NO_STORE }
+		);
+	}
+
+	return json({ status: 'ready', checks: { database: 'ok' } }, { headers: NO_STORE });
+};

tests/e2e/readyz.e2e.ts

@@ -0,0 +1,9 @@
+import { test, expect } from '@playwright/test';
+
+test('readiness endpoint reports ready when the database is reachable', async ({ request }) => {
+	const response = await request.get('/readyz');
+	expect(response.status()).toBe(200);
+	const body = await response.json();
+	expect(body.status).toBe('ready');
+	expect(body.checks.database).toBe('ok');
+});

tests/unit/readyz-endpoint.test.ts

@@ -0,0 +1,53 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+const { executeMock } = vi.hoisted(() => ({
+	executeMock: vi.fn()
+}));
+
+vi.mock('$lib/server/db', () => ({ db: { execute: executeMock } }));
+
+vi.mock('drizzle-orm', () => ({
+	sql: (strings: TemplateStringsArray) => strings.join('')
+}));
+
+// SvelteKit's generated $types is not available under vitest; the endpoint only
+// uses the type import, so stub it out.
+vi.mock('./$types', () => ({}));
+
+import { GET } from '../../src/routes/readyz/+server';
+
+// The route handler only reads request-independent state, so an empty event is fine.
+const callGet = () => GET({} as never);
+
+describe('/readyz readiness probe', () => {
+	beforeEach(() => {
+		executeMock.mockReset();
+	});
+
+	it('returns 200/ready with no-store when the database is reachable', async () => {
+		executeMock.mockResolvedValueOnce(undefined);
+
+		const response = await callGet();
+
+		expect(response.status).toBe(200);
+		expect(response.headers.get('cache-control')).toBe('no-store');
+		expect(await response.json()).toEqual({ status: 'ready', checks: { database: 'ok' } });
+	});
+
+	it('returns 503/unavailable with no-store when the database check throws', async () => {
+		const consoleError = vi.spyOn(console, 'error').mockImplementation(() => {});
+		executeMock.mockRejectedValueOnce(new Error('connection refused'));
+
+		const response = await callGet();
+
+		expect(response.status).toBe(503);
+		expect(response.headers.get('cache-control')).toBe('no-store');
+		expect(await response.json()).toEqual({
+			status: 'unavailable',
+			checks: { database: 'down' }
+		});
+		expect(consoleError).toHaveBeenCalled();
+
+		consoleError.mockRestore();
+	});
+});