Less verbose missing media logging

Instead of logging each item in the backup database that no longer has
an entry in the Plex database, consolidate it to a single line that
reports the total number of items missing.

Server/MarkerBackupManager.js

@@ -977,6 +977,8 @@ ORDER BY id DESC;`;
 
         /** @type {{ [sectionId: number]: number[] }} */
         const disconnected = {};
+        /** @type {Set<number>} */
+        const noGuid = new Set();
         for (const action of actions) {
             if (action.op === MarkerOp.Delete) {
                 continue; // Last action was a user delete, ignore it.
@@ -997,8 +999,7 @@ ORDER BY id DESC;`;
                     PlexQueries.getMovieFromGuid(action.parent_guid) :
                     PlexQueries.getEpisodeFromGuid(action.parent_guid));
                 if (!fromGuid) {
-                    Log.verbose(`No episode found for marker id ${action.id}, ` +
-                        `but keeping around in case the episode guid is added in the future.`);
+                    noGuid.add(action.id);
                     continue;
                 }
 
@@ -1012,6 +1013,12 @@ ORDER BY id DESC;`;
             }
         }
 
+        if (noGuid.size > 0) {
+            Log.verbose(`No episode found for ${noGuid.size} marker ids` +
+                (noGuid.size < 10 ? ` (${Array.from(noGuid).join(', ')})` : '') +
+                `, but keeping around in case the episode guid is added in the future.`);
+        }
+
         // Ignore disconnected markers all at once
         for (const [sectionId, markerIds] of Object.entries(disconnected)) {
             this.ignorePurgedMarkers(markerIds, parseInt(sectionId));