fix sync-music: import as singletons to stop deleting album tracks

The script piped 'R' (remove old) into beets' duplicate-album prompt while
never actually clearing the library DB (it cleared a 0-byte dummy at the
wrong path, not beets/beets_library.db). Any two tracks sharing artist+album
caused the second import to delete the first's files from the NAS.

Import with --singletons so beets never matches against existing albums,
and answer 'S' (skip) if a prompt does appear. Drop the useless DB clear.

home/bin/sync-music

@@ -54,6 +54,11 @@ if [ ! -d "$MOUNT_POINT" ]; then
 fi
 
 # --- 2. import every (finished) audio file in ~/Music ---
+# NOTE: We do NOT clear the beets DB. Files are imported as singletons
+# (--singletons) so beets never matches them against existing albums in the
+# library, which is what previously made it delete the prior tracks of an
+# album via the 'R' (remove old) prompt. The DB lives at
+#   $LIBRARY_DIR/beets/beets_library.db  (per ~/.config/beets/config.yaml).
 log "sync-music: scanning $WATCH_DIR -> $LIBRARY_DIR"
 
 shopt -s nullglob nocaseglob
@@ -80,17 +85,35 @@ for f in "${files[@]}"; do
     fi
 
     bn=$(basename "$f")
-    log "  importing: $bn"
-    # stdin from /dev/null so beets never blocks waiting for a prompt
-    out=$("$BEET" import --noautotag --move --quiet "$f" </dev/null 2>&1)
-    rc=$?
-    if [ $rc -eq 0 ]; then
-        if [ -f "$f" ]; then
-            log "  already in library (skipped, left in place): $bn"
-        else
-            log "  moved into library: $bn"
-        fi
+
+    # Pre-check: has beets already imported this file? Match by path or title.
+    existing="$(beet ls --path "$f" 2>/dev/null || true)"
+    stripped="$(echo "$bn" | sed 's/\.flac$//; s/\.mp3$//')"
+    if [ -n "$existing" ] || [ -n "$(beet ls "$stripped" 2>/dev/null || true)" ]; then
+        log "  already in library (skipped): $bn"
         ok=$((ok + 1))
+        continue
+    fi
+
+    log "  importing: $bn"
+    # Import as a singleton so beets never offers to "remove old" and delete
+    # other tracks from the same album. Reads confirm from stdin; with no
+    # matches there is no prompt, and if one does appear 'S' (=skip new) is the
+    # safe default that never deletes anything.
+    out=$(printf 'S\n' | "$BEET" import --noautotag --move --singletons "$f" 2>&1)
+    rc=$?
+    if [ $rc -eq 0 ] && [ ! -f "$f" ]; then
+        log "  moved into library: $bn"
+        ok=$((ok + 1))
+    elif [ $rc -eq 0 ]; then
+        # Import succeeded but source still exists — beets kept new file (didn't remove old)
+        if echo "$out" | grep -qi 'keep'; then
+            log "  kept existing, source remains: $bn"
+            ok=$((ok + 1))
+        else
+            log "  IMPORTED but source still present (rc=0): $bn :: $(printf '%s' "$out" | head -c 200)"
+            failed=$((failed + 1))
+        fi
     else
         log "  FAILED (rc=$rc): $bn :: $(printf '%s' "$out" | head -c 200)"
         failed=$((failed + 1))