feat(admin): delete a whitelist entry, and stop hiding a broken mail setup

Three operator gaps, all found the same way — a person could not sign up and the
app showed nobody anything.

- DELETE /api/admin/invites/{id} with a delete control on both the pending
  requests and the previously-decided list. Deny keeps the row so the person
  cannot immediately re-ask; delete forgets the address. Refused with 409 while
  a User row exists for it, because is_allowed reads that row and removing it
  would revoke a live user's access with no other sign — the list ships
  has_account so the UI withholds the button instead of offering one that fails.
- Email is fail-soft by design, which had come to mean invisible. _send() now
  records an email_undelivered notification for every admin when a message is
  skipped or rejected, naming the message that was lost, throttled to one an
  hour. /api/me carries email_configured, driving a standing warning on
  Configuration → Email / SMTP and above the access-request list.
- The sign-in card's Create account / Forgot password / Didn't get the email
  views are history entries (useHistorySubview), so Back returns to the previous
  view instead of only moving the address bar.

Also: the per-test rate-limiter reset sweeps every loaded app.* module rather
than a hand-kept list of three — this change added a limiter outside it.

frontend/src/lib/api/admin.ts

@@ -19,6 +19,9 @@ export interface Invite {
   requested_at: string | null;
   decided_at: string | null;
   decided_by: string | null;
+  /** An account already exists for this address, so the entry can't be deleted from here — deleting
+   *  it would revoke that user's access (is_allowed reads this row). Delete the user instead. */
+  has_account: boolean;
 }
 
 // Admin Configuration page: one DB-overridable setting (registry entry on the backend).
@@ -169,4 +172,5 @@ export const adminApi = {
   denyInvite: (id: number) => req(`/api/admin/invites/${id}/deny`, { method: "POST" }),
   addInvite: (email: string) =>
     req("/api/admin/invites", { method: "POST", body: JSON.stringify({ email }) }),
+  deleteInvite: (id: number) => req(`/api/admin/invites/${id}`, { method: "DELETE" }),
 };