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/api/metadata/musicBrainzApi.js

@@ -0,0 +1,43 @@
+import axiosRetry from '../../utils/retryAxios.js';
+import handleError from '../../errors/generalApiErrorHandler.js';
+
+/**
+ * Retrieves the metadata for a recording from MusicBrainz.
+ *
+ * @param {string} recordingId - The MusicBrainz recording ID (can be obtained from acoustid).
+ * @returns {Promise<Object>} - A promise resolving to the track metadata.
+ */
+export default async function getMetadata(recordingId) {
+  const baseUrl = 'https://musicbrainz.org';
+  const query = `/ws/2/recording/${recordingId}?fmt=json&inc=artists+releases+release-groups+isrcs+url-rels+discids+media+artist-credits+aliases+tags+ratings+genres`;
+
+  const { VERSION, PROJECTNAME, EMAIL } = process.env;
+  const requestOptions = {
+    headers: {
+      'User-Agent': `${PROJECTNAME}/${VERSION} ( ${EMAIL} )`,
+      Accept: 'application/json',
+    },
+  };
+
+  return axiosRetry.get(baseUrl + query, requestOptions)
+    .then((response) => {
+      const { data } = response;
+
+      if (!data) {
+        console.error(`Recording ID ${recordingId} could not retrieve metadata from MusicBrainz`);
+        return null;
+      }
+
+      console.log('Metadata retrieval successful for:', recordingId);
+      return data;
+    })
+    .catch((error) => {
+      // If it's a 404 error, log the error and proceed
+      if (error.response && error.response.status === 404) {
+        console.error(`No metadata found for recording ID: ${recordingId}`);
+        return null;
+      }
+      const errorMessage = handleError(error, recordingId);
+      throw new Error(errorMessage);
+    });
+}