Service node: a server kept running on a worker (openpanel V2)

- `service` node type: command, port, worker label, health path, env and
  weights (an artifact digest the worker fetches as {weights}). Its flow
  starts it; it restarts with backoff, reads degraded while starting or
  without a worker and down after three starts that never got healthy, and
  publishes http://<worker>:<port> once /health answers 200, "" when it ends.
- Worker protocol 3: a `serve` call, every call in its own process group,
  killed when the engine ends the call, the socket drops or the agent gets
  SIGTERM. `--service-host` overrides the address the engine sees.
- Services and compiles take no `--parallel` slot, and a flow's detail is
  built off the event loop, so a save no longer stalls the engine.

backend/fluksio/flow/controller.py

@@ -56,6 +56,7 @@ from fluksio.flow.nodes import (
     Node,
     NtfyNode,
     RbeNode,
+    ServiceNode,
     SwitchNode,
     WebSocketNode,
     WrenNode,
@@ -425,6 +426,13 @@ NODE_TYPES: dict[str, NodeType] = {
         runs_code=True,
         params_schema=_schema_of(ExecNode),
     ),
+    "service": NodeType(
+        title="Service",
+        description="Keep a server running on a worker, and publish its url.",
+        cls=ServiceNode,
+        runs_code=True,
+        params_schema=_schema_of(ServiceNode),
+    ),
     "file": NodeType(
         title="File",
         description="Read a file into the flow, or write one out of it.",