Survive a busy engine: retry, idempotent submit, resilient waiting

A driver script died of one slow answer: httpx.ReadTimeout out of
RunHandle.refresh() with a 30 s read timeout and no retry anywhere, which
cost a sweep 78 of its 84 runs.

- Split the timeout (5 s connect, 120 s read): a wrong URL fails at once,
  and a busy engine gets longer than the slowest thing it does on purpose
  (a 60 s compile, a 15 s rebuild wait).
- Retry idempotent calls three times on a transport error or 502/503/504.
  503 is the engine's own "ask again" — it is what RebuildBusy answers.
- Submit carries a key the engine stores with the run, so a retry after a
  timeout returns that run instead of starting a second. A sweep keys every
  entry, so a half-created one recreates only what is missing.
- wait() and --follow tolerate five failed polls in a row; a 404 still stops
  at once, because that is an answer rather than a gap.
- CLI says "engine not answering" and names the run still on the engine,
  instead of printing a traceback.
- runs: clamp the params column to 80 characters; events() takes the
  flow/since/until the endpoint already had; RunHandle.failures answers
  "what killed this run" from the run's own node rows.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/fluksio/models.py

@@ -340,6 +340,12 @@ class Run(SQLModel, table=True):
     group_id: str | None = Field(default=None, index=True, max_length=64)
     #: The run this one was made from, on a retry.
     parent_id: str | None = Field(default=None, max_length=64)
+    #: A key the caller made up for one submission, so retrying a submit that
+    #: may already have landed returns that run instead of starting a second.
+    #: Unique where it is set; null for anything submitted without one.
+    idempotency_key: str | None = Field(
+        default=None, unique=True, index=True, max_length=64
+    )
     #: Where it was asked for: api, cli, sdk, hook or sweep.
     cause: str = Field(default="api", max_length=32)
     #: Re-execute every node, whatever the stage cache holds for it.