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/api/routes/runs.py

@@ -368,6 +368,9 @@ RUN_COLUMNS = (
     "duration_ms",
     "seed",
     "group_id",
+    #: Set on a run made by retrying another, so a sweep that was completed
+    #: rather than repeated says which row replaced which.
+    "parent_id",
     "code_digest",
     "origin_commit",
 )
@@ -639,6 +642,20 @@ async def cancel_run(run_id: str, request: Request, session: SessionDep) -> Any:
     return run
 
 
+@router.post("/{run_id}/retry", response_model=RunRow, status_code=202)
+async def retry_run(run_id: str, request: Request, user: CurrentUser) -> Any:
+    """Run the same thing again, as a new run pointing back at this one."""
+    service = _service(request)
+    try:
+        return await run_in_threadpool(service.retry, run_id, user.email)
+    except FlowNotFound as exc:
+        raise HTTPException(status_code=404, detail=str(exc)) from exc
+    except RunRejected as exc:
+        detail = str(exc)
+        status = 404 if detail.startswith("No run ") else 422
+        raise HTTPException(status_code=status, detail=detail) from exc
+
+
 @router.delete("/{run_id}", status_code=204)
 def delete_run(run_id: str, session: SessionDep, user: CurrentUser) -> Response:
     """Forget a run and everything hanging off it.