feat(docker): per-container size+age log ring rotation [M79 step 4]

Bound docker_logs growth in the periodic cleanup task (same architecture as the
metrics/events retention). run_docker_retention gains logs_retention_days +
logs_max_bytes_per_container: it prunes lines past the age window, then keeps
only the newest ~cap bytes per (host, container) via a window-function ring
(exclusive-prefix sum, so the newest line always survives even if it alone
exceeds the cap). Containers rotate independently.

- settings DEFAULTS: docker.logs.enabled/exclude/retention_days(3)/
  max_bytes_per_container(5MB) — operator preference: ~3 days / ~5 MB
- cleanup.py reads the two windows fresh each run (rule 25, no restart)
- integration rotate test: age prune + per-container byte cap + isolation

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CAGR73DUowdVFVvYzLXC5C

steward/core/cleanup.py

@@ -76,10 +76,14 @@ async def _run_docker_retention(session, now: datetime) -> None:
         raw_days = int(await get_setting(session, "docker.retention.metrics_raw_days") or 7)
         rollup_days = int(await get_setting(session, "docker.retention.metrics_rollup_days") or 90)
         events_days = int(await get_setting(session, "docker.retention.events_days") or 30)
+        logs_days = int(await get_setting(session, "docker.logs.retention_days") or 3)
+        logs_cap = int(
+            await get_setting(session, "docker.logs.max_bytes_per_container") or 5_000_000)
         counts = await invoke_capability(
             "docker.run_retention", UserRole.viewer, session,
             events_days=events_days, metrics_raw_days=raw_days,
-            metrics_rollup_days=rollup_days, now=now,
+            metrics_rollup_days=rollup_days, logs_retention_days=logs_days,
+            logs_max_bytes_per_container=logs_cap, now=now,
         )
     if counts and any(counts.values()):
         logger.info("Docker retention: %s", counts)