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.

docs/decisions.md

@@ -158,3 +158,12 @@ says so.
 40. **`main` is release code, `dev` is development.** Work lands on `dev` and reaches `main` by merge
     at release, so `main` is always a list of things that shipped. Nightly builds, if any, come off
     `dev`.
+41. **No password minimum; rate limit logins instead.** A length policy protects against guessing,
+    and guessing is better answered directly: 10 failures per email in 15 minutes, then a 15-minute
+    lockout, cleared by a correct password. The floor was rejected because typing an 8-character
+    password on every dev account is friction with nothing behind it — there is no public
+    registration to spray, and the admin is the reset path. It was also deliberately *not* made
+    configurable: settings like this belong in code, not in an env file that grows a line per
+    preference. The limiter is keyed by email rather than IP (a proxy would mean trusting
+    `X-Forwarded-For`) and locks the *attempt rate*, not the account, so nobody can lock someone
+    else out by trying.