feat(server): add percentage-based limits to smart playlists (#5144)

* feat(playlists): add percentage-based limits to smart playlists

Add a new `limitPercent` JSON field to Criteria that allows smart playlist
limits to be expressed as a percentage of matching tracks rather than a
fixed number. For example, a playlist matching 450 songs with a 10% limit
returns 45 songs, scaling dynamically as the library grows.

When `limitPercent` is set, refreshSmartPlaylist runs a COUNT query first
to determine the total matching tracks, then resolves the percentage to an
absolute LIMIT before executing the main query. The fixed `limit` field
takes precedence when both are set. Values are clamped to [0, 100] during
JSON unmarshaling.

No database migration is needed since rules are stored as a JSON string.

* fix(criteria): validate percentage limit range in IsPercentageLimit method

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(criteria): ensure idempotency of ToSql method for expressions

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

persistence/playlist_repository.go

@@ -248,22 +248,36 @@ func (r *playlistRepository) refreshSmartPlaylist(pls *model.Playlist) bool {
 
 	// Conditionally join album/artist annotation tables only when referenced by criteria or sort
 	requiredJoins := rules.RequiredJoins()
-	if requiredJoins.Has(criteria.JoinAlbumAnnotation) {
-		sq = sq.LeftJoin("annotation AS album_annotation ON ("+
-			"album_annotation.item_id = media_file.album_id"+
-			" AND album_annotation.item_type = 'album'"+
-			" AND album_annotation.user_id = ?)", usr.ID)
-	}
-	if requiredJoins.Has(criteria.JoinArtistAnnotation) {
-		sq = sq.LeftJoin("annotation AS artist_annotation ON ("+
-			"artist_annotation.item_id = media_file.artist_id"+
-			" AND artist_annotation.item_type = 'artist'"+
-			" AND artist_annotation.user_id = ?)", usr.ID)
-	}
+	sq = r.addSmartPlaylistAnnotationJoins(sq, requiredJoins, usr.ID)
 
 	// Only include media files from libraries the user has access to
 	sq = r.applyLibraryFilter(sq, "media_file")
 
+	// Resolve percentage-based limit to an absolute number before applying criteria
+	if rules.IsPercentageLimit() {
+		// Use only expression-based joins for the COUNT query (sort joins are unnecessary)
+		exprJoins := rules.ExpressionJoins()
+		countSq := Select("count(*) as count").From("media_file").
+			LeftJoin("annotation on ("+
+				"annotation.item_id = media_file.id"+
+				" AND annotation.item_type = 'media_file'"+
+				" AND annotation.user_id = ?)", usr.ID)
+		countSq = r.addSmartPlaylistAnnotationJoins(countSq, exprJoins, usr.ID)
+		countSq = r.applyLibraryFilter(countSq, "media_file")
+		countSq = countSq.Where(rules)
+
+		var res struct{ Count int64 }
+		err = r.queryOne(countSq, &res)
+		if err != nil {
+			log.Error(r.ctx, "Error counting matching tracks for percentage limit", "playlist", pls.Name, "id", pls.ID, err)
+			return false
+		}
+		resolvedLimit := rules.EffectiveLimit(res.Count)
+		log.Debug(r.ctx, "Resolved percentage limit", "playlist", pls.Name, "percent", rules.LimitPercent, "totalMatching", res.Count, "resolvedLimit", resolvedLimit)
+		rules.Limit = resolvedLimit
+		rules.LimitPercent = 0
+	}
+
 	// Apply the criteria rules
 	sq = r.addCriteria(sq, rules)
 	insSql := Insert("playlist_tracks").Columns("id", "playlist_id", "media_file_id").Select(sq)
@@ -296,6 +310,22 @@ func (r *playlistRepository) refreshSmartPlaylist(pls *model.Playlist) bool {
 	return true
 }
 
+func (r *playlistRepository) addSmartPlaylistAnnotationJoins(sq SelectBuilder, joins criteria.JoinType, userID string) SelectBuilder {
+	if joins.Has(criteria.JoinAlbumAnnotation) {
+		sq = sq.LeftJoin("annotation AS album_annotation ON ("+
+			"album_annotation.item_id = media_file.album_id"+
+			" AND album_annotation.item_type = 'album'"+
+			" AND album_annotation.user_id = ?)", userID)
+	}
+	if joins.Has(criteria.JoinArtistAnnotation) {
+		sq = sq.LeftJoin("annotation AS artist_annotation ON ("+
+			"artist_annotation.item_id = media_file.artist_id"+
+			" AND artist_annotation.item_type = 'artist'"+
+			" AND artist_annotation.user_id = ?)", userID)
+	}
+	return sq
+}
+
 func (r *playlistRepository) addCriteria(sql SelectBuilder, c criteria.Criteria) SelectBuilder {
 	sql = sql.Where(c)
 	if c.Limit > 0 {