Close the nine open SDK tasks: one engine per directory, a tabbed dashboard, re-pairing, run recovery

serve: refuse a second engine for one data directory whatever port it was
asked for, using the pidfile and a token this directory signed. The check
runs before the database is touched and before the credential is written,
which is what left every later CLI call pointing at a dead port.

The terminal dashboard is three tabs (Overview, Runs, Logs) with the toolbar
following the focused pane, the engine's output goes to serve.log rather than
down a pipe, and closing the screen stops both reader threads so the prompt
comes back. It adopts a running engine on every start, so stop/start and
restart work on one it did not start, and a stop waits for the process to be
gone before the next start. Enrolment reports itself in the modal.

enroll: a new claim code replaces the pairing instead of being refused. The
code is redeemed before anything is written, mappings to a portal being left
are cleared, and a running engine redials when the stored enrolment changes.

runs: an engine re-queues the runs left `queued` by the one before it, and
`fluksio retry <id>` / `retry --group <sweep>` submits an interrupted run
again with the same inputs and group, recorded through Run.parent_id.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U9BoNGq6V9MdRWAte7JBuC

backend/fluksio/sdk/client.py

@@ -475,6 +475,10 @@ class Client:
         # Cancelling a cancelled run is cancelled.
         return self._call("POST", f"/runs/{run_id}/cancel", idempotent=True)
 
+    def retry(self, run_id: str) -> Any:
+        """Submit the same run again. Not idempotent: it makes a run."""
+        return self._call("POST", f"/runs/{run_id}/retry")
+
     def download(self, digest: str) -> bytes:
         response = self._request("GET", f"/artifacts/{digest}", idempotent=True)
         if response.status_code >= 400: