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.

frontend/src/components/Flow/FlowNode.tsx

@@ -16,6 +16,7 @@ import {
   Play,
   Plug,
   Radio,
+  Server,
   Shuffle,
   Split,
   Square,
@@ -58,6 +59,7 @@ const NODE_ICONS = {
   join: Merge,
   debounce: Timer,
   exec: Terminal,
+  service: Server,
   file: FileText,
   ntfy: Bell,
   wren: Bird,