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/alembic/versions/f2c6a8d15e93_run_idempotency_key.py

@@ -0,0 +1,40 @@
+"""run.idempotency_key
+
+Submitting is a POST, so a client that retries one after a timeout cannot know
+whether the first attempt landed. A key the caller mints per submission makes
+the answer a lookup: the same key returns the run it already created.
+
+Revision ID: f2c6a8d15e93
+Revises: d1f7a3c8b204
+Create Date: 2026-08-26
+
+"""
+
+import sqlalchemy as sa
+import sqlmodel.sql.sqltypes
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "f2c6a8d15e93"
+down_revision = "d1f7a3c8b204"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.add_column(
+        "run",
+        sa.Column(
+            "idempotency_key",
+            sqlmodel.sql.sqltypes.AutoString(length=64),
+            nullable=True,
+        ),
+    )
+    # Nullable and unique: every run submitted without a key stays NULL, and
+    # both SQLite and Postgres allow as many of those as there are runs.
+    op.create_index("ix_run_idempotency_key", "run", ["idempotency_key"], unique=True)
+
+
+def downgrade():
+    op.drop_index("ix_run_idempotency_key", table_name="run")
+    op.drop_column("run", "idempotency_key")