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 = {