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/ConfigPanel.tsx

@@ -6,6 +6,8 @@ import { Plug, RotateCcw, Send } from "lucide-react";
 import { api, type ConfigItem, type PlexTestResult } from "../lib/api";
 import { notify } from "../lib/notifications";
 import Tooltip from "./Tooltip";
+import InlineNotice from "./InlineNotice";
+import { useMe } from "../lib/useMe";
 import Tabs, { usePersistedTab } from "./Tabs";
 import { LoadingState, StateMessage } from "./QueryState";
 import { PageToolbar } from "./PageShell";
@@ -35,6 +37,7 @@ const GROUP_ORDER = [
 export default function ConfigPanel() {
   const { t } = useTranslation();
   const qc = useQueryClient();
+  const me = useMe();
   const q = useQuery({ queryKey: qk.adminConfig(), queryFn: api.adminConfig });
   const data = q.data;
 
@@ -204,7 +207,11 @@ export default function ConfigPanel() {
             </div>
 
             {activeGroup === "email" && (
-              <div className="mt-3 pt-3 border-t border-border/60">
+              <div className="mt-3 pt-3 border-t border-border/60 space-y-3">
+                {/* The one place an operator can see that mail is off. Sending is fail-soft, so a
+                    half-filled config costs verification links, resets and access-request notices
+                    with no error anywhere — say so next to the fields that fix it. */}
+                {!me.email_configured && <InlineNotice>{t("config.emailOff")}</InlineNotice>}
                 <Tooltip hint={t("config.testEmailHint")}>
                   <button
                     onClick={() => testEmail.mutate()}