[mob][photos] Social (#8616)

## Description
 
Comment and like on photos/videos in shared albums.

Author: ua741

mobile/apps/photos/lib/db/files_db.dart

@@ -587,6 +587,56 @@ class FilesDB with SqlDbBase {
     return ids;
   }
 
+  /// Checks which (fileID, collectionID) pairs exist in the database.
+  ///
+  /// Returns a set of keys in the format "collectionID_fileID" for files that exist.
+  /// This is more efficient than calling getUploadedFile() for each pair.
+  Future<Set<String>> getExistingFileKeys(
+    Set<(int fileID, int collectionID)> pairs,
+  ) async {
+    if (pairs.isEmpty) return {};
+
+    final db = await instance.sqliteAsyncDB;
+    final existingKeys = <String>{};
+
+    // Build a query with OR conditions for each pair
+    // SQLite has a limit on compound SELECT statements, so we batch
+    const batchSize = 100;
+    final pairsList = pairs.toList();
+
+    for (var i = 0; i < pairsList.length; i += batchSize) {
+      final batch = pairsList.sublist(
+        i,
+        i + batchSize > pairsList.length ? pairsList.length : i + batchSize,
+      );
+
+      final conditions = batch
+          .map(
+            (_) => '($columnUploadedFileID = ? AND $columnCollectionID = ?)',
+          )
+          .join(' OR ');
+
+      final args = <Object>[];
+      for (final (fileID, collectionID) in batch) {
+        args.add(fileID);
+        args.add(collectionID);
+      }
+
+      final results = await db.getAll(
+        'SELECT $columnUploadedFileID, $columnCollectionID FROM $filesTable WHERE $conditions',
+        args,
+      );
+
+      for (final row in results) {
+        final fileID = row[columnUploadedFileID] as int;
+        final collectionID = row[columnCollectionID] as int;
+        existingKeys.add('${collectionID}_$fileID');
+      }
+    }
+
+    return existingKeys;
+  }
+
   Future<(Set<int>, Map<String, int>)> getUploadAndHash(
     int collectionID,
   ) async {