feat: Refactor project structure for modularity and expandability

- Implemented the service and adapter layers instead of controllers to simplify the integration of new audio recognition and metadata fetching APIs.
- Unified code styling and practices throughout the project to ensure consistency.

src/utils/validateAudioFiles.js

@@ -0,0 +1,34 @@
+import fs from 'fs/promises';
+import path from 'path';
+
+/**
+ * Validates whether a given file path points to a supported audio file.
+ * Logs an error for each file that is not an audio file or not a file at all and ignores it.
+ *
+ * @param {string} filePath - The absolute path to the file to validate.
+ * @returns {Promise<string|null>} - The file path if it is a supported audio file, otherwise null.
+ */
+export default async function validateAudioFile(filePath) {
+  try {
+    // Ensure that the path points to a file
+    const stats = await fs.lstat(filePath);
+    if (!stats.isFile()) {
+      console.error(`The path ${filePath} is not an audio file and is ignored.`);
+      return null; // Stop further checks and return null
+    }
+
+    // List of supported audio file extensions
+    const audioExtensions = ['.mp3', '.wav', '.aac', '.flac', '.ogg', '.aiff', '.m4a'];
+
+    // Check if the file extension is in the list of supported audio formats
+    const fileExtension = path.extname(filePath).toLowerCase();
+    if (!audioExtensions.includes(fileExtension)) {
+      console.error(`File ${path.basename(filePath)} is not an audio file and is ignored.`);
+      return null; // Not an audio file, return null
+    }
+
+    return filePath; // The file is a supported audio file
+  } catch (e) {
+    console.error(`Error validating file ${filePath}: ${e}`);
+  }
+}