feat(agent): add read-only webhook payload fetch endpoint for backfill

backend/tests/test_openapi_agent_webhook_payload_endpoint.py

@@ -0,0 +1,15 @@
+# ruff: noqa: INP001, S101
+
+from __future__ import annotations
+
+from app.main import app
+
+
+def test_openapi_includes_agent_webhook_payload_read_endpoint() -> None:
+    schema = app.openapi()
+
+    path = "/api/v1/agent/boards/{board_id}/webhooks/{webhook_id}/payloads/{payload_id}"
+    assert path in schema["paths"]
+    op = schema["paths"][path]["get"]
+    tags = set(op.get("tags", []))
+    assert "agent-worker" in tags