fix(robomp): deferred rate-limited submissions

Stored a bounded per-login overflow backlog and promoted deferred events oldest-first as rolling-window capacity became available. Surfaced the deferred state through the dashboard contract and documented admission behavior.

Fixes #5882

python/robomp/tests/test_queue_dispatch.py

@@ -92,3 +92,25 @@ async def test_dispatch_pr_synchronize_is_noop(
     await _make_pool(settings, db)._dispatch(_pr_row("synchronize"))  # noqa: SLF001
 
     assert called is False
+
+
+@pytest.mark.asyncio
+async def test_claim_promotes_deferred_submission_after_window_frees(settings: Settings, db: Database) -> None:
+    assert db.record_submission(delivery_id="accepted", login="alice", repo="octo/widget")
+    assert db.defer_submission_event(
+        delivery_id="deferred",
+        event_type="issues",
+        login="alice",
+        repo="octo/widget",
+        issue_key="octo/widget#8",
+        payload={"action": "opened", "issue": {"number": 8}},
+        cap=1,
+        reason="rate limit",
+    )
+    settings.rate_limit_window_seconds = -1
+
+    row = await _make_pool(settings, db)._claim_next_unique()  # noqa: SLF001
+
+    assert row is not None
+    assert row.delivery_id == "deferred"
+    assert row.state == "running"