Audio and video are processed off the request

Every audio and video upload was probed and remuxed inside the request, whatever its length; ffmpeg would read any
protocol and probe any format; `-map 0` kept the data tracks iPhones add, which mp4 refuses; nothing was ever
transcoded, so HEVC or MPEG-4 Part 2 reached browsers that can't play them, and the advertised video_matrix_limit
and frame rate limit were never applied; the output was read whole into memory, the video was saved before its
poster could fail, and the poster's frame leaked in /tmp. FLAC uploads were served as 404.

Now an upload sent to /api/v2/media is stored as sent in media-incoming (beside the media root, never served) and
answered with 202 and no url, while a ProcessMedia job, one at a time, does the work; GET /api/v1/media/:id answers 206
until it is ready, or 422 with why, and media still processing can't be posted. v1 processes before answering.
ffmpeg reads only that file (protocol whitelist, format forced from the probe) and drops data and subtitle tracks. A
video browsers play as it is (H.264, VP8, VP9, AV1 within Media:MaxVideoPixels and MaxFrameRate) is remuxed, anything
else transcoded to H.264 that fits, as Mastodon does; longer than Media:MaxSeconds is refused. Outputs move into place
only once everything succeeded, every temporary file goes, durations are kept, FLAC is served as audio/flac, and the
unit gets PrivateTmp. The instance API advertises the limits that are now applied.

No pasture scenario uploads audio or video through PrivaPub, so the sweep could not see this. MastodonMediaTests: v2
answers 202 then the job makes it playable (and an unreadable file 422 once processed), media still processing can't
be posted, MPEG-4 Part 2 becomes H.264, a video over the limit is made smaller, FLAC is served.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

deploy/systemd/privapub.service

@@ -14,6 +14,8 @@ Environment=ASPNETCORE_ENVIRONMENT=Production
 Environment=DOTNET_CLI_TELEMETRY_OPTOUT=1
 Restart=always
 RestartSec=5
+# its own /tmp: ffmpeg's and the uploads' temporary files are nobody else's
+PrivateTmp=true
 SyslogIdentifier=privapub
 
 [Install]