fix(metrics): time-series rows get time-ordered ids, so an insert stops costing a disk read (#5149)

Measured on dev (385a579), agent posts cut to five samples still dropped:
flush=8.8s for 990 rows, 6.6s for 680, 8.4s for 1000 — about 110 rows/s,
~9 ms a row, with rules and evaluation near zero. One random read per
row fits a random uuid4 primary key whose index no longer fits in memory:
every new row lands on a random page of it. (Inferred from the rate; the
table and index sizes have not been read.)

- steward.core.ids.time_ordered_id: a UUIDv7-layout string (48-bit ms
  timestamp, then random), same 36-char shape as the stored uuid4 ids, so
  no migration. Python's uuid7() arrives in 3.14; the image runs 3.13.
- Used for plugin_metrics and its hourly rollup, monitor_results,
  audit_events, the docker metrics/hourly/events/logs tables, and the
  traefik and unifi time series.
- MAX_SAMPLES_PER_INGEST 5 → 2 (~400 rows) for now: it fits the agent's
  10 s even at the old rate, so hosts land data whichever way the new rate
  comes out. Raise it once the new rate is measured.

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

plugins/host_agent/routes.py

@@ -343,7 +343,9 @@ SLOW_INGEST_SECONDS = 2.0
 # instance that did not finish inside the agent's 10 s, the rollback left the
 # buffer full, and every later post failed the same way. The newest samples
 # are kept: the older ones would be lost to the agent's ring buffer anyway.
-MAX_SAMPLES_PER_INGEST = 5
+# Two (~400 rows) fits 10 s even at the ~110 rows/s measured before
+# plugin_metrics got time-ordered ids; raise it once the new rate is known.
+MAX_SAMPLES_PER_INGEST = 2
 
 
 def newest_samples(samples: list, limit: int) -> tuple[list, int]: