fix(downloads): an upload's conversion is guarded, counted, named and cleaned up

- Every ffmpeg/ffprobe over a library file names the demuxers it may use
  (edit.INPUT_GUARD; a downloaded thumbnail edit.THUMB_GUARD): a playlist-type
  demuxer would read local paths named inside a user's upload.
- A re-encode ends on an even frame (edit.EVEN_FRAME): an odd 4:4:4/RGB source
  failed its conversion; a clip stores the frame its file has.
- An upload counts against its owner's cap from the moment it lands (queued, or
  kept after a failed conversion) — the footprint, the GC's cap and the admin
  view read one ON_DISK predicate — and a conversion that outgrows the cap is
  refused, keeping the original.
- The converted name keeps _a{id} (a dotted bare name once lost it and two
  uploads shared a file); a converted file placed but not committed is removed,
  and the orphan sweep enters .uploads; the LRU cap never evicts an upload.
- Ingest judges the extension the file is stored under; a failed conversion is
  convert_failed / upload_missing, reports the "converting" phase, and notifies
  as an upload. Job statuses in routes/downloads use JobStatus.

backend/app/downloads/edit.py

@@ -194,6 +194,44 @@ def clip_duration(spec: dict, source_duration: int | None) -> int | None:
 # only patchily (the one safe H.264 is 8-bit 4:2:0).
 X264_ARGS = ["-c:v", "libx264", "-preset", "veryfast", "-crf", "20", "-pix_fmt", "yuv420p"]
 AAC_ARGS = ["-c:a", "aac", "-b:a", "192k"]
+# The last step of every video filter chain a libx264 re-encode runs: 4:2:0 halves the chroma, so
+# libx264 refuses an odd width or height ("width not divisible by 2") — measured on an odd source
+# that is not 4:2:0 already (a 4:4:4 or RGB screen capture, 1365x767). A crop of a 4:2:0 source is
+# rounded to its chroma grid by ffmpeg itself; this holds every other path to the encoder to the same
+# even frame. Rounding down to even costs at most one pixel row/column.
+EVEN_FRAME = "scale=trunc(iw/2)*2:trunc(ih/2)*2"
+
+# The demuxers ffmpeg/ffprobe may use to open a library file — single-file containers only. Without
+# it ffmpeg picks the demuxer from the file's CONTENT and NAME, and an upload's name is the user's:
+# a playlist-type demuxer (HLS, DASH, concat) opens whatever local paths its entries name, so a
+# crafted upload could copy another user's clip or a Plex file into its own converted asset. A file
+# no demuxer here can open fails the probe, and an unprobeable upload is a download-only 'file'.
+MEDIA_DEMUXERS = (
+    "mov",  # mp4, m4a, mov, 3gp
+    "matroska",  # mkv, webm
+    "mpegts",
+    "mpeg",
+    "avi",
+    "asf",
+    "flv",
+    "ogg",
+    "mp3",
+    "aac",
+    "ac3",
+    "eac3",
+    "flac",
+    "wav",
+    "w64",
+    "aiff",
+    "caf",
+    "amr",
+)
+INPUT_GUARD = ["-format_whitelist", ",".join(MEDIA_DEMUXERS)]
+# A downloaded thumbnail is the remote site's bytes under the suffix yt-dlp gave it (`.png`, `.webp`,
+# `.jpg` — `pipeline._THUMB_SUFFIXES`): only the single-image demuxers those open, so a "thumbnail"
+# that is really a playlist cannot pull a local file into the poster. Anything else fails, and the
+# poster falls back to a cut frame (`ensure_poster`).
+THUMB_GUARD = ["-format_whitelist", "png_pipe,jpeg_pipe,webp_pipe"]
 
 
 def build_edit_cmd(src: Path, dest: Path, spec: dict, out_ext: str) -> list[str]:
@@ -221,7 +259,7 @@ def build_edit_cmd(src: Path, dest: Path, spec: dict, out_ext: str) -> list[str]
     ]
     if start:
         cmd += ["-ss", f"{start:.3f}"]
-    cmd += ["-i", str(src)]
+    cmd += [*INPUT_GUARD, "-i", str(src)]
     if end is not None:
         dur = end - start
         if dur > 0:
@@ -229,10 +267,9 @@ def build_edit_cmd(src: Path, dest: Path, spec: dict, out_ext: str) -> list[str]
 
     if reencode:
         # Input -ss + re-encode is frame-accurate (decodes from the prior keyframe, discards up to
-        # the exact cut). A crop adds the filter; an accurate trim re-encodes with no filter.
-        if crop:
-            cmd += ["-vf", f"crop={crop['w']}:{crop['h']}:{crop['x']}:{crop['y']}"]
-        cmd += [*X264_ARGS, *AAC_ARGS]
+        # the exact cut). A crop adds the filter; an accurate trim only evens the frame.
+        cropf = f"crop={crop['w']}:{crop['h']}:{crop['x']}:{crop['y']}," if crop else ""
+        cmd += ["-vf", cropf + EVEN_FRAME, *X264_ARGS, *AAC_ARGS]
     else:
         cmd += ["-c", "copy", "-avoid_negative_ts", "make_zero"]
 
@@ -280,11 +317,14 @@ def build_concat_plan(src: Path, dest: Path, spec: dict, out_ext: str, staging:
         parts, labels = [], []
         for i, s in enumerate(segs):
             st, en = s["start_s"], s["end_s"]
-            parts.append(f"[0:v]trim=start={st}:end={en},setpts=PTS-STARTPTS{cropf}[v{i}]")
+            parts.append(
+                f"[0:v]trim=start={st}:end={en},setpts=PTS-STARTPTS{cropf},{EVEN_FRAME}[v{i}]"
+            )
             parts.append(f"[0:a]atrim=start={st}:end={en},asetpts=PTS-STARTPTS[a{i}]")
             labels.append(f"[v{i}][a{i}]")
         parts.append(f"{''.join(labels)}concat=n={len(segs)}:v=1:a=1[v][a]")
         cmd = head + [
+            *INPUT_GUARD,
             "-i",
             str(src),
             "-filter_complex",
@@ -304,7 +344,9 @@ def build_concat_plan(src: Path, dest: Path, spec: dict, out_ext: str, staging:
             lines.append(f"inpoint {s['start_s']}")
             lines.append(f"outpoint {s['end_s']}")
         prep[listp] = "\n".join(lines) + "\n"
-        cmd = head + ["-f", "concat", "-safe", "0", "-i", str(listp), "-c", "copy"]
+        # The list is ours; the whitelist reaches the file it names too (ffmpeg hands it down).
+        guard = ["-format_whitelist", "concat," + INPUT_GUARD[1]]
+        cmd = head + ["-f", "concat", "-safe", "0", *guard, "-i", str(listp), "-c", "copy"]
 
     if out_ext in ("mp4", "m4a", "mov"):
         cmd += ["-movflags", "+faststart"]
@@ -477,7 +519,7 @@ def run_ffprobe_json(
     package: the shared media probe (`probe.probe_json`: the stored facts and the phone-safe rule) goes
     through it, so the
     subprocess flags/timeout/error handling live in one place instead of drifting copies."""
-    args = ["ffprobe", "-v", "error"]
+    args = ["ffprobe", "-v", "error", *INPUT_GUARD]
     if show_streams:
         args.append("-show_streams")
     if show_format:
@@ -507,6 +549,7 @@ def probe_duration(path: Path) -> int:
                 "ffprobe",
                 "-v",
                 "error",
+                *INPUT_GUARD,
                 "-show_entries",
                 "format=duration",
                 "-of",
@@ -564,6 +607,7 @@ def build_storyboard_cmd(src: Path, dest: Path, duration_s: int | None, geom: di
         "-loglevel",
         "error",
         "-nostdin",
+        *INPUT_GUARD,
         "-i",
         str(src),
         "-frames:v",
@@ -587,6 +631,7 @@ def build_poster_cmd(src: Path, dest: Path, at_s: float) -> list[str]:
         "-nostdin",
         "-ss",
         f"{max(0.0, at_s):.3f}",
+        *INPUT_GUARD,
         "-i",
         str(src),
         "-frames:v",
@@ -608,6 +653,7 @@ def build_thumbnail_jpg_cmd(src: Path, dest: Path) -> list[str]:
         "-loglevel",
         "error",
         "-nostdin",
+        *THUMB_GUARD,
         "-i",
         str(src),
         "-frames:v",