fix(downloads): a download larger than the space left says so, not "processing failed"

yt-dlp refuses a file announced over `max_filesize` by logging one line and returning with no file
(measured, and pinned in test_formats against a local server): the worker found no output and
failed the job as `postprocess`. `formats.watching_size_cap` now sees that line through our yt-dlp
logger, and the download (and, should it ever happen, a YouTube clip's range fetch) fails as the
owner's refusal with DOWNLOAD_TOO_BIG_MESSAGE, classified `quota_exceeded`. The download's quota
copy says it does not fit in what is left — true at a full cap too — instead of "Storage full".
A pre-existing type error the change touched is fixed (baseline down by one).

backend/tests/test_formats.py

@@ -3,6 +3,13 @@ playability guarantee) and must SELECT H.264 (not merely prefer it), so shared d
 every browser incl. iOS/Safari. Non-mp4 containers and non-H.264 vcodecs are coerced, not honoured.
 See the iPad AV1 broken-player fix + the format narrowing."""
 
+import contextlib
+import functools
+import http.server
+import threading
+
+import yt_dlp
+
 from app.downloads import edit as editmod
 from app.downloads import formats
 
@@ -162,3 +169,40 @@ def test_the_mp3_fix_moved_the_audio_cache_identity(monkeypatch):
     current = formats.format_sig(audio)
     monkeypatch.setattr(formats, "_AUDIO_SIG_VERSION", 4)
     assert formats.format_sig(audio) != current
+
+
+class _QuietFiles(http.server.SimpleHTTPRequestHandler):
+    def log_message(self, *args):  # the test's output is yt-dlp's, not the server's
+        pass
+
+    def handle(self):
+        # yt-dlp hangs up on a file over the cap as soon as it reads its size.
+        with contextlib.suppress(ConnectionError):
+            super().handle()
+
+
+def test_yt_dlp_refuses_a_file_over_the_cap_without_raising_and_the_watch_sees_it(tmp_path):
+    # The boundary `watching_size_cap` reads, held against the yt-dlp this image pins: over
+    # `max_filesize` its HTTP downloader logs one line and RETURNS — no exception, no file — so
+    # without the watch the pipeline would only find "no output file" and call it a processing fault.
+    served = tmp_path / "served"
+    served.mkdir()
+    (served / "v.mp4").write_bytes(b"\0" * 200_000)
+    srv = http.server.ThreadingHTTPServer(
+        ("127.0.0.1", 0), functools.partial(_QuietFiles, directory=str(served))
+    )
+    threading.Thread(target=srv.serve_forever, daemon=True).start()
+    url = f"http://127.0.0.1:{srv.server_port}/v.mp4"
+    try:
+        for cap, refused in ((1_000, True), (10_000_000, False)):
+            out = tmp_path / f"out-{cap}"
+            opts = formats.build_ydl_opts(
+                {}, str(out / "%(id)s.%(ext)s"), lambda d: None, max_bytes=cap
+            )
+            with formats.watching_size_cap() as over, yt_dlp.YoutubeDL(opts) as ydl:
+                ydl.extract_info(url, download=True)
+            assert over[0] is refused, cap
+            files = [p for p in out.glob("*") if p.suffix == ".mp4"] if out.exists() else []
+            assert bool(files) is not refused, cap
+    finally:
+        srv.shutdown()