path instead of failing the video transcode
Previously IMAGE_CODECS contained the raster ffprobe codec names ('png',
'mjpeg', 'jpeg', 'webp', 'gif', 'tiff', 'bmp', 'jpegls') but not 'svg'.
An SVG-as-asset (e.g. an architecture diagram dragged into a project) was
correctly tagged media_type='image' in the DB but ffprobe reported its
codec as 'svg', which fell through to the video branch, found
durationMs===null, and died with 'Empty or truncated source: codec=svg,
resolution=0x0'. That clogs the failed-jobs list with red rows that have
nothing to do with broken captures.
Two fixes here:
1) Add 'svg' to IMAGE_CODECS so the existing transcodeImage()/poster
path handles it.
2) Also bail to the poster path when the asset row itself says
media_type='image', even if ffprobe didn't return a codec name we
recognize (defensive — catches future formats like AVIF without
requiring an explicit catalog update).
Closes part of #13.