Login was returning 200 + correct user JSON + writing a row to the
sessions table, but emitting zero Set-Cookie headers. Root cause:
session.regenerate() → set fields → session.save() → res.json()
Calling session.save() manually writes the store but bypasses
express-session's res.end() hook, which is the only path that adds
the Set-Cookie header to the response. The cookie was never sent to
the browser even though the session existed server-side — hence the
redirect loop.
Fix: remove the manual save(). Set the session fields and call
res.json() directly inside regenerate()'s callback; express-session
handles store write + Set-Cookie automatically on res.end().
e71c330bdd