fix(downloads): harden upload serving + ingest against stored-XSS and leaks (E7 review)

Deep-review remediation for the E7 upload path, all findings adversarially verified:

- Serve every library file with a Content-Type chosen from the ffprobe-verified media_kind
  + a strict container allow-list (storage.served_media_type), never guessed from the
  user-controlled file extension. Closes a stored-XSS vector where a media/HTML polyglot
  uploaded as "evil.html" was served inline as text/html on the login-free /watch surface.
  Applied to both the authenticated /file and the public watch_file endpoints.
- Classify uploaded still images (jpg/png/gif/webp/…) as media_kind='file' so they are
  force-downloaded and don't get a broken in-app video player.
- Roll back the on-disk side of an upload when the ingest transaction fails after place_file
  (unlink the placed file + its deterministic poster path), so a failed commit can't orphan
  bytes in the GC-skipped .uploads tree.
- Floor download_upload_max_bytes at min=1 so a stored 0 can't silently disable the per-file cap.
- Dedupe the ffprobe-JSON invocation into edit.run_ffprobe_json (shared by probe_codecs and
  upload._probe) and the upload extension into storage._clean_upload_ext; drop the now-unused
  parse_probe_streams.

Tests: public + authenticated force-download and the polyglot-not-served-as-HTML guard, the
real _probe classifier (image/cover-art/dims/duration), the ingest rollback/cleanup path, the
max_jobs branch, the cap-floor invariant, and run_ffprobe_json error handling.

backend/app/downloads/edit.py

@@ -438,18 +438,42 @@ _BROWSER_SAFE_VCODEC = ("h264",)
 _BROWSER_SAFE_ACODEC = ("aac", "mp3")
 
 
-def parse_probe_streams(payload: str) -> tuple[str | None, str | None]:
-    """(video_codec, audio_codec) out of ffprobe's JSON stream list, lowercased.
-
-    JSON, not the flat `default=nw=1` output: there the fields arrive in ffprobe's own struct
-    order, which puts `codec_name` BEFORE `codec_type`, so a line-by-line "remember the last
-    codec_type" parser attributes every name to the PREVIOUS stream. Measured on real downloads:
-    an ordinary H.264+AAC mp4 probed as `v=aac a=None` — the browser-playability check then
-    believed the video stream was AAC and re-encoded a perfectly fine file on every download."""
+def run_ffprobe_json(
+    path: Path,
+    *,
+    show_streams: bool = False,
+    show_format: bool = False,
+    entries: str | None = None,
+) -> dict | None:
+    """One ffprobe call → its parsed JSON dict, or None on any failure (timeout, non-zero exit,
+    missing binary, unparseable output). The SINGLE ffprobe-JSON-invocation point in the downloads
+    package: the browser-playability codec probe here and the upload-ingest probe (`upload._probe`)
+    both go through it, so the subprocess flags/timeout/error handling live in one place instead of
+    two drifting copies."""
+    args = ["ffprobe", "-v", "error"]
+    if show_streams:
+        args.append("-show_streams")
+    if show_format:
+        args.append("-show_format")
+    if entries:
+        args += ["-show_entries", entries]
+    args += ["-of", "json", str(path)]
     try:
-        streams = json.loads(payload or "{}").get("streams") or []
-    except ValueError:
-        return None, None
+        out = subprocess.run(args, capture_output=True, text=True, timeout=30, check=True).stdout
+        return json.loads(out)
+    except (subprocess.TimeoutExpired, subprocess.CalledProcessError, OSError, ValueError):
+        return None
+
+
+def _codecs_from_streams(streams: list) -> tuple[str | None, str | None]:
+    """(video_codec, audio_codec) lowercased from an ffprobe JSON stream list — the first of each kind.
+
+    Parsed from the JSON `streams` list, NOT ffprobe's flat `default=nw=1` output: there the fields
+    arrive in ffprobe's own struct order, which puts `codec_name` BEFORE `codec_type`, so a line-by-line
+    "remember the last codec_type" parser attributes every name to the PREVIOUS stream. Measured on real
+    downloads: an ordinary H.264+AAC mp4 probed as `v=aac a=None` — the browser-playability check then
+    believed the video stream was AAC and re-encoded a perfectly fine file on every download. Reading each
+    stream dict by its own `codec_type` is immune to that ordering."""
     vcodec = acodec = None
     for s in streams:
         name = (s.get("codec_name") or "").lower() or None
@@ -464,26 +488,10 @@ def parse_probe_streams(payload: str) -> tuple[str | None, str | None]:
 def probe_codecs(path: Path) -> tuple[str | None, str | None]:
     """Return (video_codec, audio_codec) lowercased via one ffprobe call; None for a missing
     stream or if ffprobe fails (a failed probe is treated as 'leave it alone' upstream)."""
-    try:
-        out = subprocess.run(
-            [
-                "ffprobe",
-                "-v",
-                "error",
-                "-show_entries",
-                "stream=codec_type,codec_name",
-                "-of",
-                "json",
-                str(path),
-            ],
-            capture_output=True,
-            text=True,
-            timeout=30,
-            check=True,
-        ).stdout
-    except (subprocess.TimeoutExpired, subprocess.CalledProcessError, OSError):
+    data = run_ffprobe_json(path, entries="stream=codec_type,codec_name")
+    if data is None:
         return None, None
-    return parse_probe_streams(out)
+    return _codecs_from_streams(data.get("streams") or [])
 
 
 def probe_duration(path: Path) -> int: