Merge develop into feature/50-upload-soundtrack

The API spec is now formatted like before (2-space indents, sorted keys):
develop's spec with the upload paths and types added, so the diff to
develop is only the upload API.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/integration/test_scene_split.gd

@@ -226,7 +226,7 @@ func test_click_on_top_bar_while_quiz_popup_is_open_only_closes_it() -> void:
 
 func test_sync_disables_quiz_buttons_until_finished() -> void:
 	_server.respond_bytes("/sync", PackedByteArray())
-	_server.respond_json("/sync/progress", {"progress": 10, "time_spent": "00:00:01"})
+	_server.respond_json("/sync/progress", {"progress": 10, "time_spent": "00:01"})
 	var sync_popup: BasePopup = _window.get_node("SyncPopup")
 
 	_top_bar_button("SyncButton").pressed.emit()