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/login.html

@@ -0,0 +1,21 @@
+{{define "content"}}
+<h1>Kirjaudu</h1>
+
+{{with .Data.Errors.form}}<p class="error">{{.}}</p>{{end}}
+
+<form method="post" action="/login" class="stack">
+  <label>Sähköposti
+    <input type="email" name="email" value="{{.Data.Email}}" required autofocus autocomplete="email">
+  </label>
+  <label>Salasana
+    <input type="password" name="password" required autocomplete="current-password">
+  </label>
+  <label class="row">
+    <input type="checkbox" name="remember" value="1"> Pysy kirjautuneena 30 päivää
+  </label>
+  <button type="submit">Kirjaudu</button>
+</form>
+
+<p class="muted">Levyraati on kutsuvierasklubi. Kutsukoodilla pääset mukaan
+  <a href="/register">tästä</a>.</p>
+{{end}}