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/components/NotificationsPanel.tsx

@@ -243,6 +243,7 @@ function NotificationRow({
   const isMaintenance = n.type === "maintenance" && typeof n.data?.count === "number";
   const isScheduler = n.type === "scheduler" && typeof n.data?.job_id === "string";
   const isDownloadDone = n.type === "download_done";
+  const isEmailUndelivered = n.type === "email_undelivered" && typeof n.data?.reason === "string";
   let title = n.title;
   let body = n.body;
   if (isMaintenance) {
@@ -258,6 +259,17 @@ function NotificationRow({
       job,
     });
     body = d.summary || n.body;
+  } else if (isEmailUndelivered) {
+    // Email is fail-soft: a drop leaves no error anywhere else, so this notice IS the trace. Name
+    // the message that was lost, since that is what tells the operator what their users didn't get.
+    const d = n.data as { reason: string; subject?: string };
+    title = t("inbox.emailUndelivered.title");
+    body = t(
+      d.reason === "not_configured"
+        ? "inbox.emailUndelivered.notConfigured"
+        : "inbox.emailUndelivered.sendFailed",
+      { subject: d.subject || n.body || "" },
+    );
   } else if (isDownloadDone) {
     // "Download ready" / "Edit ready" per the producer's kind, with the video's title as the body
     // (shipped in data.title). Same notification type; the copy differs so an edit isn't mislabelled.