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.

backend/app/downloads/edit.py

@@ -54,6 +54,10 @@ class EditAborted(Exception):
 
 # --- edit spec ------------------------------------------------------------------------------
 
+# SEC-13: ceiling on a join's cut-list. An accurate join emits two filters per segment into a single
+# ffmpeg `filter_complex`, so "however many the caller sent" is an unbounded command line.
+_MAX_SEGMENTS = 100
+
 
 def normalize_edit_spec(spec: dict | None) -> dict:
     """Canonicalize a raw edit spec: clamp/round trim, coerce crop to ints, drop empties.
@@ -88,6 +92,12 @@ def normalize_edit_spec(spec: dict | None) -> dict:
             if en > st:
                 norm.append({"start_s": round(st, 3), "end_s": round(en, 3)})
         norm.sort(key=lambda x: x["start_s"])
+        # SEC-13: an accurate join builds one ffmpeg `filter_complex` with two filters per segment,
+        # so an unbounded list becomes an unbounded command line and an ffmpeg that never finishes.
+        # The cap is far above any real cut-list (the UI's own splitter produces a handful) and the
+        # excess is DROPPED rather than raising: the surplus segments are the attack, the first
+        # _MAX_SEGMENTS are a legitimate edit.
+        del norm[_MAX_SEGMENTS:]
         if norm:
             out["segments"] = norm
             # A join always chooses a codec path: accurate=filter-concat re-encode,