♻️(backend) replace hardcoded file extensions with Python enum

Convert hardcoded string file extensions into a well-defined Python enum.
Improves type safety and centralizes extension definitions for better
maintainability and consistency across the codebase.

It was dirty manipulating literals for file extension validation …

src/backend/core/recording/enums.py

@@ -0,0 +1,10 @@
+"""Enums related to recordings."""
+
+from enum import Enum
+
+
+class FileExtension(Enum):
+    """Enum for file extensions used in recordings."""
+
+    OGG = "ogg"
+    MP4 = "mp4"