Add write-through cache for per-item track/episode data

lidarr_cleanup.sh and sonarr_cleanup.sh already walk every artist/series
individually (trackFile/episodefile) for their own cleanup decisions --
that walk now also writes the raw per-item data through to a short-lived
tmpfs cache (arr_item_cache_write, 4h freshness, no persistent backup
since every consumer already has its own live fallback). lidarr_missing_art.sh
runs later in the same nightly window and now reads that cache first,
skipping its own redundant per-artist walk entirely on a hit. Sonarr side
is write-through only for now -- no second consumer exists yet, but the
data's there for whenever one does. Future consumers: arr_get_cached_items()
first, live per-item fetch as fallback, same pattern as these two.

Arrs_Stack/lidarr_cleanup.sh

@@ -329,12 +329,20 @@ TRACKED_FILE="$TMP_DIR/tracked_paths.txt"
 # Fetches every artist's track-file paths fresh into TRACKED_FILE/TRACKED_MAP/TRACKED_COUNT.
 # Pulled into a function so the rescan-aware retry below can re-fetch after waiting without
 # duplicating this whole loop inline.
+#
+# Write-through — also caches the raw per-track data via arr_item_cache_write() (2026-07-17)
+# so scripts running later in the same maintenance window (lidarr_missing_art.sh) can read it
+# instead of repeating this same per-artist walk. This walk is happening regardless for our
+# own cleanup decisions; the cache write is free by comparison. See common.sh for the pattern.
 _fetch_tracked_files() {
     > "$TRACKED_FILE"
+    local all_tracks_tmp
+    all_tracks_tmp=$(mktemp)
     while IFS= read -r artist_id; do
         [[ -z "$artist_id" ]] && continue
         ARTIST_TRACKS=$(arr_api "$LIDARR_URL" "$LIDARR_API_KEY" "v1" "trackFile?artistId=${artist_id}" "Lidarr" 2>/dev/null)
         if [[ -n "$ARTIST_TRACKS" ]]; then
+            echo "$ARTIST_TRACKS" >> "$all_tracks_tmp"
             while IFS= read -r api_path; do
                 [[ -z "$api_path" ]] && continue
                 translate_path "$api_path" >> "$TRACKED_FILE"
@@ -342,6 +350,9 @@ _fetch_tracked_files() {
         fi
     done <<< "$ARTIST_IDS"
 
+    arr_item_cache_write "lidarr" "$(jq -s 'add // []' "$all_tracks_tmp" 2>/dev/null)"
+    rm -f "$all_tracks_tmp"
+
     sort -u "$TRACKED_FILE" -o "$TRACKED_FILE"
 
     # Build in-memory lookup map — O(1) per lookup vs O(n) grep per file