perf(host_agent): aggregate metric history in SQL; DISTINCT ON for latest

Follow-on to the plugin_metrics indexes. plugin_metrics is already retention-
bounded (core cleanup prunes > data.retention_days, default 90d) and charts top
out at 30d, so the cost wasn't growth — it was the read path shipping raw rows
to Python.

- _history_for_host: bucket + average in SQL via date_bin (epoch-aligned, ~120
  buckets) instead of fetching every raw sample (a 30d range was hundreds of
  thousands of rows) and downsampling in Python. Uses the new
  (source_module, resource_name, recorded_at) index.
- _latest_metrics_for_host: DISTINCT ON (resource_name, metric_name) ORDER BY
  recorded_at DESC — newest row per group in one index-ordered pass, replacing
  the GROUP-BY-max subquery self-joined back to the whole history.
- Integration test validates both against Postgres.

Deliberately not a materialized raw→hourly rollup: these query-side changes
deliver the speed; a rollup would additionally cut storage and remains a future
option if scale demands it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016Jg27rgypiW2efULXJDtMC

tests/integration/test_host_metrics.py

@@ -0,0 +1,80 @@
+"""Integration: host-metrics read paths (DISTINCT ON latest + SQL date_bin history).
+
+Validates the two host_agent query helpers that back the slow host views, against
+a live Postgres: the latest-per-(resource,metric) lookup and the bucket-averaged
+history (which now aggregates in SQL instead of shipping raw rows to Python).
+Requires STEWARD_DATABASE_URL.
+"""
+from __future__ import annotations
+
+import asyncio
+import os
+import uuid
+from datetime import datetime, timedelta, timezone
+
+import pytest
+
+pytestmark = pytest.mark.integration
+
+_NEEDS_DB = pytest.mark.skipif(
+    not os.environ.get("STEWARD_DATABASE_URL"),
+    reason="integration test needs a live Postgres (STEWARD_DATABASE_URL)",
+)
+
+
+@pytest.fixture
+def app():
+    if not os.environ.get("STEWARD_DATABASE_URL"):
+        pytest.skip("needs Postgres")
+    from steward.app import create_app
+    return create_app(testing=False)
+
+
+@_NEEDS_DB
+def test_latest_distinct_on_and_sql_bucketed_history(app):
+    from sqlalchemy import text
+    from steward.models.metrics import PluginMetric
+    from plugins.host_agent.routes import (
+        SOURCE_MODULE, _history_for_host, _latest_metrics_for_host,
+    )
+
+    now = datetime.now(timezone.utc)
+    hostname = "metrics-host-" + uuid.uuid4().hex[:8]
+
+    async def _go():
+        async with app.db_sessionmaker() as s:
+            async with s.begin():
+                await s.execute(
+                    text("DELETE FROM plugin_metrics WHERE resource_name LIKE :p"),
+                    {"p": hostname + "%"},
+                )
+                rows = []
+                # Host-level CPU samples, oldest → newest (10, 20, 30).
+                for i, val in enumerate([10.0, 20.0, 30.0]):
+                    rows.append(PluginMetric(
+                        source_module=SOURCE_MODULE, resource_name=hostname,
+                        metric_name="cpu_pct", value=val,
+                        recorded_at=now - timedelta(minutes=30 - i * 10),
+                    ))
+                # A sub-resource (root mount) to exercise the host:% match.
+                rows.append(PluginMetric(
+                    source_module=SOURCE_MODULE, resource_name=hostname + ":/",
+                    metric_name="disk_used_pct", value=80.0,
+                    recorded_at=now - timedelta(minutes=5),
+                ))
+                s.add_all(rows)
+
+            latest = await _latest_metrics_for_host(s, hostname)
+            hist = await _history_for_host(s, hostname, now - timedelta(hours=1))
+            return latest, hist
+
+    latest, hist = asyncio.run(_go())
+
+    # DISTINCT ON returns the newest sample per (resource, metric).
+    assert latest[hostname]["cpu_pct"] == 30.0
+    assert latest[hostname + ":/"]["disk_used_pct"] == 80.0
+
+    # SQL date_bin aggregation returns cpu_pct buckets; averages stay in range.
+    cpu = hist["cpu_pct"]
+    assert cpu, "expected bucketed cpu_pct history"
+    assert all(10.0 <= v <= 30.0 for _, v in cpu)