fix(downloads,sync): bound what one user can make the server fetch, download and spend

SEC-12: the download box accepts an arbitrary http(s) URL and yt-dlp fetches
it from the WORKER, so any approved user could aim the server at the compose
network, the LAN or a cloud metadata endpoint — and read the result off
job.error, where "connection refused" vs "HTTP Error 401" vs "timed out" is a
port scanner. downloads/urlguard.py refuses a URL whose host resolves to a
loopback/private/link-local/reserved address, at enqueue AND again in the
worker (DNS can move in between, and the worker is where the socket opens).
A network-class failure on a `url` job now reports one fixed sentence instead
of the raw transport text; the error CODE is still classified from the
original, so the localized explanation is unchanged. DNS rebinding is
explicitly NOT closed — that needs the connection gated inside yt-dlp — and
the module says so rather than implying more than it does.

SEC-13: `noplaylist` only demotes a video-inside-a-playlist URL to that video,
so a URL that is purely a playlist or channel still expanded into every
entry — one enqueue, unbounded downloads. `playlist_items` pins it to one.
Size was unbounded too: `check_enqueue` compares the CURRENT footprint against
the cap, and a download's size is unknown until it lands, so one job could
overshoot by any amount; the owner's remaining budget is now handed to yt-dlp
as `max_filesize`, applied AFTER the adapter merge so no source adapter can
raise a limit the quota layer set. A join's cut-list is capped at 100: an
accurate join emits two filters per segment into one ffmpeg filter_complex.

SEC-14: the YouTube budget is instance-wide, so one user holding down a
manual sync spent the day for everyone. Per-user (not per-IP — the cost
belongs to the account) 3-per-10-minutes limiters on the two bulk pulls,
subscriptions and playlists, with separate budgets so one does not close the
other. The review also listed /rss, /backfill and /enrich; E3 deleted them,
and search stays unlimited on purpose — it is interactive, not a bulk pull.

Gate: pytest 896 -> 924 (none skipped), vitest 671.

frontend/src/lib/api/core.ts

@@ -72,6 +72,14 @@ function localizeDetail(d: unknown): string {
   if (o?.code === "upload_too_large")
     return t("downloads.errors.upload.tooLarge", { size: formatBytes(o.limit) });
   if (o?.code === "upload_empty") return t("downloads.errors.upload.empty");
+  // SEC-12: a download URL the server refuses to fetch. The reasons are deliberately distinct —
+  // "we could not look that host up" is a different thing to tell someone than "we will not fetch
+  // from that address" — so an unknown reason falls back to the generic refusal rather than a key.
+  if (o?.code === "url_blocked")
+    return t(`downloads.errors.urlBlocked.${o.reason}`, t("downloads.errors.urlBlocked.generic"));
+  // SEC-14: a manual sync trigger on cooldown. Not an error the user caused so much as a pace
+  // limit, and the copy says so.
+  if (o?.code === "sync_cooldown") return t("errors.syncCooldown");
   return t("errors.generic");
 }