refactor(dashboard): split api.py into handler modules

dashboard/handlers/__init__.py

@@ -0,0 +1,7 @@
+"""Handler mixin modules for the Echo Task Board API.
+
+Each module exposes a mixin class whose methods plug into
+`TaskBoardHandler` (defined in dashboard/api.py). This keeps
+api.py as a thin HTTP router while each concern lives in its
+own small module.
+"""