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/app/downloads/formats.py

@@ -105,6 +105,28 @@ def log_safe(text: object) -> str:
     return _PATH_RE.sub("…", text)
 
 
+# yt-dlp's HTTP downloader, refusing a file announced larger than `max_filesize`, logs this line and
+# RETURNS — no exception and no file (measured; test_formats holds it against the pinned yt-dlp).
+_OVER_CAP_LINE = "File is larger than max-filesize"
+_OVER_CAP: contextvars.ContextVar[list[bool] | None] = contextvars.ContextVar(
+    "formats_over_cap", default=None
+)
+
+
+@contextlib.contextmanager
+def watching_size_cap() -> Iterator[list[bool]]:
+    """While yt-dlp works inside it: a one-slot flag, set when it refused a file over the size cap
+    (`build_ydl_opts(max_bytes=…)`). It then returns as if it had downloaded, and all the caller
+    would find is no file — a processing fault, to anyone reading it, for what is the owner's
+    storage. A context variable, like `hiding`: yt-dlp's logger calls run on the extraction's thread."""
+    flag = [False]
+    token = _OVER_CAP.set(flag)
+    try:
+        yield flag
+    finally:
+        _OVER_CAP.reset(token)
+
+
 class _YdlLog:
     """yt-dlp's logger protocol, pointed at ours.
 
@@ -124,6 +146,8 @@ class _YdlLog:
     is the one line whose level should say so."""
 
     def _emit(self, level: int, msg: str) -> None:
+        if _OVER_CAP_LINE in msg and (over := _OVER_CAP.get()) is not None:
+            over[0] = True
         # The line goes as an ARGUMENT, not as the format string. yt-dlp writes progress as
         # `[download]  53.2% of 12.00MiB`, and while stdlib logging only applies `%` formatting to
         # a record that has args, this way the question never arises — and it is the shape ruff's