fix(api): a write that hit a connection failure is still reported, and every failure is reported once

Deep-review round 4, the "one report per failure" contract round 3 introduced.

- `req` marked gateway (502/503/504) and network failures "already reported" because they raise
  the connection notice. But that notice is about connectivity and removes itself on the very next
  answer — which every caller's own resync refetch fetches at once — so a delete that hit a
  keepalive reset flashed the notice and was then reported nowhere, the list still there. Only the
  durable reports (the error dialog, the 401 sign-out) count now; the caller says the action failed.
- A connection notice the user closed swallowed every later one for the rest of the visit; it is
  raised again.
- `req` and `uploadReq` share one failure policy (`reportFailure`). The upload copy had drifted: a
  502 opened the blocking "Server error (502)" dialog and cleared a notice that was still true.
- A taken Plex collection name is a structured 409, localized, and the create/rename calls are
  quiet: it was an English dialog plus a Hungarian toast. `notifyCollectionEditError` is gone.
- The mutation onErrors that toasted unconditionally after a non-quiet request (invite approve /
  deny / add, demo whitelist, audit clear, scheduler, share, full history, edit, config tests, tag
  attach) go through `notifyActionError`; the playlist delete's hand-rolled copy of it too (it takes
  a level now). The user role / suspend / delete actions had no onError at all: a user deleted in
  another tab failed silently and stayed listed — now said, and the list refetched.

frontend/src/components/Scheduler.tsx

@@ -25,6 +25,7 @@ import { useConfirm } from "./ConfirmProvider";
 import { relativeTime } from "../lib/format";
 import { notify } from "../lib/notifications";
 import Tooltip from "./ui/Tooltip";
+import { notifyActionError } from "../lib/actionErrors";
 
 const POLL_MS = 4000;
 // While any job is running, poll faster so short scheduled runs aren't missed between ticks.
@@ -375,14 +376,14 @@ export default function Scheduler() {
   const pauseResume = useMutation({
     mutationFn: (paused: boolean) => (paused ? api.resumeSync() : api.pauseSync()),
     onSuccess: () => qc.invalidateQueries({ queryKey: qk.scheduler() }),
-    onError: () => notify({ level: "error", message: t("scheduler.toggleFailed") }),
+    onError: (e) => notifyActionError(e, t("scheduler.toggleFailed")),
   });
 
   const intervalMut = useMutation({
     mutationFn: (v: { jobId: string; minutes: number }) =>
       api.updateSchedulerJob(v.jobId, v.minutes),
     onSuccess: () => qc.invalidateQueries({ queryKey: qk.scheduler() }),
-    onError: () => notify({ level: "error", message: t("scheduler.intervalFailed") }),
+    onError: (e) => notifyActionError(e, t("scheduler.intervalFailed")),
   });
 
   // Manual "run now" triggers. The wrapper runs in a background thread server-side; the