fix: correctly handle NULL library_id in search service

Updates SearchMediaItemsUnified to conditionally set LibraryID parameter
only when it's valid. Previously, the code always set LibraryID in the
dbParams struct, which caused pgx to pass a zero UUID instead of NULL
to PostgreSQL.

New behavior:
  - Only sets dbParams.LibraryID when params.LibraryID.Valid is true
  - When library_id is empty, LibraryID is omitted from the struct
  - Go's zero value + pgx's "field not set" detection = NULL in SQL

Also fixes type mismatches in SearchFieldValues method where
SearchQuery parameter needed explicit pgtype.Text wrapping with
Valid=true flag for proper nullable text handling.

This ensures that omitting the library_id query parameter results in
searching across all libraries, not filtering by zero UUID.

internal/services/search.go

@@ -66,7 +66,6 @@ func (s *SearchService) SearchMediaItemsUnified(ctx context.Context, params Sear
 	// Build database parameters
 	dbParams := database.SearchMediaItemsUnifiedParams{
 		UserID:         params.UserID,
-		LibraryID:      params.LibraryID,
 		AuthorFilter:   pgtype.Text{String: params.AuthorFilter, Valid: true},
 		SeriesFilter:   pgtype.Text{String: params.SeriesFilter, Valid: true},
 		GenreFilter:    pgtype.Text{String: params.GenreFilter, Valid: true},
@@ -82,6 +81,10 @@ func (s *SearchService) SearchMediaItemsUnified(ctx context.Context, params Sear
 		Offset:         pgtype.Int4{Int32: int32(params.Offset), Valid: true},
 	}
 
+	if params.LibraryID.Valid {
+		dbParams.LibraryID = params.LibraryID
+	}
+
 	// Execute unified search query
 	results, err := s.db.SearchMediaItemsUnified(ctx, dbParams)
 	if err != nil {
@@ -115,7 +118,7 @@ func (s *SearchService) SearchFieldValues(ctx context.Context, params FieldSearc
 	switch params.FieldType {
 	case "author":
 		results, err := s.db.SearchAuthorValues(ctx, database.SearchAuthorValuesParams{
-			SearchQuery: params.SearchQuery,
+			SearchQuery: pgtype.Text{String: params.SearchQuery, Valid: true},
 			UserID:      params.UserID,
 			LibraryID:   params.LibraryID,
 			Limit:       pgtype.Int4{Int32: int32(params.Limit), Valid: true},
@@ -132,7 +135,7 @@ func (s *SearchService) SearchFieldValues(ctx context.Context, params FieldSearc
 
 	case "genre":
 		results, err := s.db.SearchGenreValues(ctx, database.SearchGenreValuesParams{
-			SearchQuery: params.SearchQuery,
+			SearchQuery: pgtype.Text{String: params.SearchQuery, Valid: true},
 			UserID:      params.UserID,
 			LibraryID:   params.LibraryID,
 			Limit:       pgtype.Int4{Int32: int32(params.Limit), Valid: true},
@@ -149,7 +152,7 @@ func (s *SearchService) SearchFieldValues(ctx context.Context, params FieldSearc
 
 	case "series":
 		results, err := s.db.SearchSeriesValues(ctx, database.SearchSeriesValuesParams{
-			SearchQuery: params.SearchQuery,
+			SearchQuery: pgtype.Text{String: params.SearchQuery, Valid: true},
 			UserID:      params.UserID,
 			LibraryID:   params.LibraryID,
 			Limit:       pgtype.Int4{Int32: int32(params.Limit), Valid: true},
@@ -166,7 +169,7 @@ func (s *SearchService) SearchFieldValues(ctx context.Context, params FieldSearc
 
 	case "language":
 		results, err := s.db.SearchLanguageValues(ctx, database.SearchLanguageValuesParams{
-			SearchQuery: params.SearchQuery,
+			SearchQuery: pgtype.Text{String: params.SearchQuery, Valid: true},
 			UserID:      params.UserID,
 			LibraryID:   params.LibraryID,
 			Limit:       pgtype.Int4{Int32: int32(params.Limit), Valid: true},