Add member accounts: invites, registration, login, sessions, ban

Step 2 of the build order. The admin mints an invite link, the recipient
registers with it, and from then on has a session.

- The invite is spent in the same transaction that creates the account, so a
  failed signup leaves the code usable
- Sessions are idle timeouts, 24h or 30 days with remember me, read from a
  cookie or a bearer header, extended at most once a minute
- Ban is a reversible toggle that drops the member's live sessions
- No password minimum; login is rate limited instead, 10 failures per email
  in 15 minutes, cleared by a correct password
- Invite codes render as links carrying ?code=, which the register form
  prefills; PUBLIC_URL makes them pasteable from the loopback admin panel

Tests cover invite spending, the idle timeout, ban, and the rate limiter.

templates/home.html

@@ -0,0 +1,5 @@
+{{define "content"}}
+<h1>Jono</h1>
+<p class="muted">Jono on tyhjä — kappaleita ei vielä voi lähettää. Tämä sivu täyttyy kun
+  lähetysputki ja arvostelut ovat valmiit.</p>
+{{end}}