fix(downloads): a produced file is held to the space left by its real size

yt-dlp's `max_filesize` only stops a plain HTTP file that announces its size: YouTube's fragmented
formats never meet it — measured on localdev, a 718 MB download ran to the end with 1 MB left and
put its owner four times over the cap. The download now re-reads what is left once its file is made
(after the phone-safe pass) and refuses one larger than that as the owner's refusal; an editor clip,
which nothing checked, is held the same way, as a player clip already was.

backend/app/downloads/formats.py

@@ -307,7 +307,9 @@ def build_ydl_opts(
 
     `max_bytes` (SEC-13) is the caller's remaining storage budget; yt-dlp's HTTP downloader refuses
     a file announced larger than it rather than filling the disk and only then being caught by the
-    quota accounting. Its ffmpeg downloader (a `download_ranges` fetch) does not read it."""
+    quota accounting. Its ffmpeg downloader (a `download_ranges` fetch) does not read it, nor does
+    a fragmented download (YouTube's formats: measured, 718 MB fetched with 1 MB left) — so the
+    pipeline also holds every produced file to what is left by its real size."""
     s = normalize(spec)
     h = s["max_height"]
     opts: dict = {

backend/app/downloads/pipeline.py

@@ -563,6 +563,13 @@ def _download(
             CONVERTING,
             int(info["duration"]) if info.get("duration") else None,
         )
+        # Held to the cap by its real size, as a clip is: yt-dlp's `max_filesize` only stops a plain
+        # HTTP file that announces its size, and YouTube's fragmented formats never meet it
+        # (measured: 718 MB fetched with 1 MB left). Read again — other jobs may have landed.
+        with SessionLocal() as qdb:
+            room = quota.remaining_bytes(qdb, user_id)
+        if room is not None and produced.stat().st_size > room:
+            raise _OwnerRefused(dlerrors.DOWNLOAD_TOO_BIG_MESSAGE)
         ext = produced.suffix.lstrip(".").lower()
         # Codecs/dims/duration come from a PROBE of the produced file — the ground truth — NOT yt-dlp's
         # `info` dict, whose fields are absent for some sources (Facebook among them): deriving
@@ -871,6 +878,13 @@ def _process_edit(job_id: int, asset_id: int) -> None:
         # playable as its source: hold it to the same rule as every other ready file.
         dest_staging = _ensure_phone_safe(job_id, dest_staging, staging, "editing", total_s)
         out_ext = dest_staging.suffix.lstrip(".").lower() or out_ext
+        # New storage on the owner's footprint, held to what is left by its real size, as a
+        # download and a player clip are.
+        if user_id is not None:
+            with SessionLocal() as qdb:
+                room = quota.remaining_bytes(qdb, user_id)
+            if room is not None and dest_staging.stat().st_size > room:
+                raise StatedFailure(dlerrors.CLIP_TOO_BIG_MESSAGE)
 
         rel = storage.edit_rel_path(user_id, asset_id, out_ext)
         storage.place_file(dest_staging, settings.download_root, rel)