The save endpoint coerced library_track_id with int(), which rejected
every non-numeric id with "Invalid library track id". Library ids are
str(ratingKey) — numeric for Plex but GUIDs/hashes for Navidrome,
Jellyfin, and other Subsonic servers — and are stored in the TEXT
tracks.id column, so the coercion broke manual matching on every
non-Plex server.
Replace the int() coercion with a normalize_library_track_id() helper
that trims and rejects only empty input, passing the opaque string id
straight through. Plex numeric ids are unaffected (SQLite INTEGER
affinity still stores a clean numeric string as an int, so existing
matches are byte-identical) and no schema migration is needed (the
INTEGER column already stores non-numeric ids as text).
Tests: pure-helper cases (numeric/GUID/whitespace/empty) plus a real-DB
round-trip proving a GUID id saves, reads back unchanged, and enriches.
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