Closes#572 (rhwc).
Navidrome has no API for setting an artist image — it reads
`artist.jpg` (or `folder.jpg`) from the artist folder during
library scans. SoulSync's `update_artist_poster` for Navidrome
was a no-op, so users only ever saw album-art-derived thumbnails
as the artist photo.
- new "Write Artist Image" button on artist detail page
- POST /api/artist/<id>/write-image-to-disk derives the artist
folder from any track's resolved file_path (reuses
_resolve_library_file_path so docker mount translation +
library.music_paths probes from #558 apply), fetches the photo
from the configured metadata source priority chain, downloads
with content-type validation, writes atomically via
`<filename>.tmp + os.replace`
- when active server is Navidrome, triggers a library scan
immediately so the file is picked up
- respects existing artist.jpg (frontend prompts before
overwriting) so user-supplied photos aren't clobbered
- works for plex / jellyfin too as a fallback layer — both
servers also read artist.jpg from disk
26 tests pin the pure helpers in core/library/artist_image.py:
folder derivation (trailing sep / empty / non-string), URL
picking (missing attr / whitespace / non-string), download
(non-image content-type / 404 / timeout / empty body), atomic
write (replace / temp-cleanup-on-failure / overwrite guard /
missing folder).
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