feat(workflow): enhance message stages and image routing

backend/services/message_policy.py

@@ -0,0 +1,20 @@
+"""Shared completion policies for deterministic Message stages."""
+
+from typing import Literal
+
+
+MESSAGE_INTERRUPTIBLE = "interruptible"
+MESSAGE_PLAYBACK = "playback"
+MESSAGE_CONFIRMATION = "confirmation"
+MESSAGE_COMPLETION_POLICIES = frozenset(
+    {
+        MESSAGE_INTERRUPTIBLE,
+        MESSAGE_PLAYBACK,
+        MESSAGE_CONFIRMATION,
+    }
+)
+MessageCompletionPolicy = Literal[
+    "interruptible",
+    "playback",
+    "confirmation",
+]