refactor: use extensible design for fetching playlist from single file

app/playlistscout.h

@@ -0,0 +1,31 @@
+// SPDX-FileCopyrightText: 2026 yyc12345 <yyc12321@outlook.com>
+//
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include <QUrl>
+#include <QStringList>
+#include <QList>
+#include <optional>
+
+namespace PlaylistScout {
+    /// The result of scout
+    struct ScoutResult {
+        /// The final fetched playlist (it can be empty).
+        /// Each item must be a QUrl pointing to a local file.
+        QList<QUrl> playlist;
+        /// The index of given file located in playlist.
+        /// Nullopt if it is not presented in playlist.
+        std::optional<qsizetype> indexHint;
+    };
+
+    /// @brief Scout available image files in given file's parent dir with given allowed suffixes.
+    /// @param[in] url The path to file whose parent dir is our target dir for finding image files.
+    /// @param[in] allowedSuffixes 
+    /// Each items in this list is a wildcard pattern for matching allowed file name, like \c *.png .
+    /// The fetched item will be preserved if its name matching any of them.
+    /// If this list is empty, there is no restrictions for finding files.
+    /// @return Scout result if success, otherwise nullopt.
+    std::optional<ScoutResult> scout(const QUrl &url, const QStringList &allowedSuffixes);
+}