feat(traffic): a host's top senders, on its page and as get_host_senders (#6115)

The host page gains a "Who's sending" panel, and the MCP a tool, that
put the host's outbound (or inbound) traffic down to its containers and
NFS mounts: read in shared time buckets, so the host's busiest bucket
says who was sending in it. A swarm service's tasks add up under its
name; what none of them accounts for is the host itself.

Also fixes CI 9488: per-container net_rx_bps / net_tx_bps join the
docker alert catalog, so a rule can catch a container flooding the
uplink.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/core/formatting.py

@@ -361,8 +361,8 @@ METRICS: dict[str, tuple[str, str]] = {
     "maintenance_failed": ("Maintenance failed", "1 when the host's latest scheduled or manual update failed or timed out, or the host didn't come back into service and still isn't"),
     "temp_c_max": ("Hottest sensor", "the host's hottest real thermometer; inputs with nothing wired to them are left out"),
     "temp_c": ("Sensor temperature", "one temperature sensor (resource host:temp:label)"),
-    "net_rx_bps": ("Network in", "bytes per second received, by the whole host or one interface"),
-    "net_tx_bps": ("Network out", "bytes per second sent, by the whole host or one interface"),
+    "net_rx_bps": ("Network in", "bytes per second received, by the whole host, one interface or one container"),
+    "net_tx_bps": ("Network out", "bytes per second sent, by the whole host, one interface or one container"),
     "disk_read_bps": ("Disk reads", "bytes per second read, from all disks or one"),
     "disk_write_bps": ("Disk writes", "bytes per second written, to all disks or one"),
     "drives_failing": ("Drives failing", "drives on the host failing their own health check, raising a critical warning or out of spare blocks"),