Add Lidarr tracked-data cache + duplicate artist cleanup

Shared cache (lidarr_get_tracked_data() in common.sh) so scripts stop
hitting Lidarr's live API for tracked counts every run, and stop
treating a mid-rescan dip as a genuine problem — a whole-library
RescanFolders legitimately makes trackFileCount read far below normal
while it re-verifies every file (confirmed 2026-07-16: 22% of normal
mid-scan). Cache reads fresh-if-recent, waits out an active rescan
(calibrated to that command's own historical duration, tracked per
command name since RescanFolders and DownloadedAlbumsScan take wildly
different amounts of time), then falls back to a stale cache rather
than hard-failing after a few strikes.

lidarr_cleanup.sh: no longer stacks a fresh DownloadedAlbumsScan on
top of one already running, and the tracked-count floor check now
waits out a genuine rescan instead of aborting on every overlap.

lidarr_duplicate_artist_cleanup.sh (new): finds case-insensitive
duplicate artist entries — same display name, different MusicBrainz
ID, added when a search/list-sync matches the wrong same-named artist.
Deletes the empty phantom side and blocks it from Import List
Exclusions, leaves genuinely-different-real-artists alone (checked by
album title overlap, deduped per-artist first so a legitimate reissue
under an artist's own catalog doesn't false-flag as cross-artist
overlap), and only notifies for the rare case where both sides have
real, overlapping content.

lidarr_cache_prefill.sh (new): warms the cache at array start so nothing
reads it cold after boot.

lidarr_missing_art.sh, lidarr_release_fixer.sh: write-through the cache
as a side effect of fetches they already needed for their own purposes.

Arrs_Stack/lidarr_missing_art.sh

@@ -255,6 +255,9 @@ echo "━━━ $ICON_SYNC Building Album Directory Map ━━━"
 
 declare -A ALBUM_DIR_MAP
 _artist_list=$(curl_json "$LIDARR_URL/api/v1/artist?apikey=$LIDARR_API_KEY")
+# Write-through — keeps the shared tracked-data cache fresh as a side effect of a fetch
+# this script already needed for its own purposes. See lidarr_get_tracked_data() in common.sh.
+lidarr_cache_write "$_artist_list"
 _map_artist_count=$(echo "$_artist_list" | jq '. | length')
 info "Fetching track files for $_map_artist_count artists..."