feat(server): add workbench API, DSH planner worker and RAG pipeline

server/app/execution_config.py

@@ -0,0 +1,18 @@
+"""Execution backend selection.
+
+inline: legacy mode; FastAPI spawns a local background thread.
+dsh: workbench only creates CampaignRun/TaskQueueItem rows; an external DSH (default)
+worker leases and executes the content production pipeline.
+"""
+from __future__ import annotations
+
+import os
+
+
+def execution_backend() -> str:
+    value = os.getenv("YAWAN_EXECUTION_BACKEND", "dsh").strip().lower()
+    return "dsh" if value == "dsh" else "inline"
+
+
+def is_dsh_backend() -> bool:
+    return execution_backend() == "dsh"