test: add comprehensive e2e test suite with shared helpers and DB seeding

Extract shared test runner (helpers.js), add file-based SQLite with
setup_db.py for fixture seeding, and add tests for auth guard, credentials
management, full registration flow, health endpoint, password auth, and
magic link registration errors. 66 checks across 7 test files.

tests/e2e/test_health.js

@@ -0,0 +1,13 @@
+// tests/e2e/test_health.js
+// Smoke test: health endpoint returns OK.
+
+const { TARGET_URL, run } = require('./helpers');
+
+run(async (page, assert) => {
+  console.log('\n--- Health endpoint ---');
+  const resp = await page.request.get(`${TARGET_URL}/health`);
+  assert(resp.ok(), `Health endpoint returns 200 (status: ${resp.status()})`);
+
+  const body = await resp.json();
+  assert(body.status === 'ok', `Health response is {"status":"ok"} (got: ${JSON.stringify(body)})`);
+});