fix(host_agent): a post that reached Steward counts as the agent seen, even if storing it failed (#5514)

At 20:25-20:39Z the NFS storage under steward_db slowed badly (checkpoint
sync 4.6 s, then 70.7 s with one file at 21.9 s, against the usual 0.002 s).
Ingest commits stalled past the agent's 10 s timeout and were dropped, and
last_seen_at, written in that same transaction, went with them. Agents
posting every few seconds read as silent for 3 minutes, and "Agent offline"
fired and emailed for all three hosts.

Ingest now notes each agent's arrival in memory right after the (read-only)
registration lookup, before any write; seconds_since_seen counts from the
later of last_seen_at and that arrival.

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

plugins/host_agent/routes.py

@@ -24,6 +24,7 @@ from steward.core.time_range import parse_range, RANGE_OPTIONS
 from steward.models.hosts import Host
 from steward.models.metrics import PluginMetric
 from .models import HostAgentRegistration
+from .scheduler import arrivals
 # Query helpers live in a model-free module so integration tests can import them
 # without the plugin-loader double-registration guard tripping (see metrics_query).
 from .metrics_query import (
@@ -242,6 +243,8 @@ async def ingest():
             )).scalar_one_or_none()
             if host is None:
                 return _error(401, "invalid_token")
+            # The agent reached us: that holds even if storing its post fails (#5514).
+            arrivals(current_app)[reg.host_id] = received_at
 
             accepted = 0
             metric_rows: list[PluginMetric] = []