A /health endpoint for the Docker health checks

The health checks of web, seed-api and seed-relay called /api/v1/stats
every 30 s, whose release statistics query kept about a core busy.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/functional/web_api.spec.ts

@@ -135,3 +135,11 @@ test.group('Web API', (group) => {
     assert.isTrue(row.par2Only)
   })
 })
+
+test.group('Health check', () => {
+  test('/health answers when the database does', async ({ client }) => {
+    const response = await client.get('/health')
+    response.assertStatus(200)
+    response.assertBody({ ok: true })
+  })
+})