feat(users): invite by email — a one-time link where the invitee makes their own account (milestone 496)

The operator's ask: an invite link sent by mail, the key in the URL, and
the new user choosing their own username, password and email.

- user_invites (migration 0037): the key's sha256 only, the address, the
  role and days the admin chose, who sent it, used/revoked.
- core/invites: create (revokes an older open invite to the address),
  renew (new key, new clock), find_pending, the four states, signup checks.
- /invite/<key>, outside sign-in and throttled like login: the invitee's
  username, email (prefilled, theirs to change) and password; the account
  takes the admin's role, the key is used up in the same transaction and
  they are signed in. A bad key gets one plain message.
- Settings › Users: Invite by email (address, role, 1/7/30 days), sent from
  the admin in their zone; without a mail server, or if sending fails, the
  link is shown once to copy. Invites not yet used are listed with Resend
  and Withdraw.
- Audit: user.invited, user.invite_resent, user.invite_revoked, user.joined.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/static/steward.css

@@ -451,6 +451,14 @@ a.stat-tile:hover { border-color: var(--border-mid); background: var(--bg-elevat
 .form-section > .section-title { margin-bottom: 0.9rem; }
 .form-pair { display: grid; grid-template-columns: repeat(auto-fit, minmax(min(14rem, 100%), 1fr)); gap: 0 1rem; }
 .form-pair[hidden] { display: none; }
+/* The invite page (milestone 496): the login page's centred card, with labelled fields. */
+.invite-page { max-width: 420px; margin: 4rem auto; }
+.invite-title { font-family: var(--font-serif); font-size: 1.5rem; font-weight: 500; margin: 0 0 0.5rem; }
+.invite-lede { color: var(--text-muted); margin: 0 0 1.25rem; }
+.invite-submit { width: 100%; margin-top: 0.25rem; }
+/* A link shown once to copy (an invite without mail): the value and its Copy button. */
+.copy-once { display: flex; gap: 0.5rem; align-items: center; }
+.copy-once .input { flex: 1; font-family: ui-monospace, monospace; font-size: 0.8rem; }
 /* A column of checkboxes: a target's groups, a group's members. */
 .check-list { display: grid; gap: 0.4rem; }
 .ansible-fields { padding: 0.9rem 1rem 0.25rem; margin-bottom: 0.75rem; border: 1px solid var(--border); border-radius: var(--fs-radius-md); background: var(--bg-elevated); }