feat(metrics): roll plugin_metrics up to hourly to bound storage

plugin_metrics grows by (sources × resources × ~30s cadence); keeping 90d of raw
is a large table. Add a raw→hourly rollup (mirroring the Docker plugin) so only a
short raw window is kept at full resolution, with hourly averages archived longer.

- PluginMetricHourly model + core migration 0024 (plugin_metrics_hourly: avg/max/
  count per source/resource/metric/hour, unique bucket constraint + lookup index).
- steward/core/metrics_retention.rollup_plugin_metrics: date_trunc('hour') agg of
  raw older than the hour-aligned raw window, idempotent pg upsert into hourly,
  delete the rolled raw, prune hourly beyond the rollup window.
- cleanup.py: plugin_metrics is no longer blanket-deleted at data.retention_days;
  _run_metrics_retention drives the rollup with windows read live from settings.
- Settings: metrics.retention.raw_days (7) + rollup_days (90), tunable on the
  Thresholds & Retention page (new "Host metrics retention" card).
- Chart read: _history_for_host merges the hourly rollup (older part of the range)
  with raw date_bin (recent part, capped ≤1h), so 30d charts keep working —
  recent at full resolution, older at hourly. Route passes raw_days from settings.
- Tests: unit (cutoff helpers) + integration (rollup aggregates/prunes; history
  merges hourly + raw) against Postgres.

Speed was already handled by the indexes + SQL aggregation; this is the storage
lever (raw window ~10x smaller).

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

steward/models/metrics.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 import uuid
 from datetime import datetime, timezone
-from sqlalchemy import DateTime, Float, Index, String
+from sqlalchemy import DateTime, Float, Index, Integer, String, UniqueConstraint
 from sqlalchemy.orm import Mapped, mapped_column
 from .base import Base
 
@@ -28,3 +28,30 @@ class PluginMetric(Base):
         Index("ix_plugin_metrics_module_resource_metric_recorded",
               "source_module", "resource_name", "metric_name", "recorded_at"),
     )
+
+
+class PluginMetricHourly(Base):
+    """Hourly rollup of plugin_metrics — the coarse series that retention rolls
+    raw samples into before pruning them, so multi-day/week history stays cheap.
+
+    One row per (source_module, resource_name, metric_name, hour bucket); the
+    unique constraint is the conflict target for the idempotent rollup upsert.
+    Charts read this for the part of a range older than the raw-retention window.
+    """
+    __tablename__ = "plugin_metrics_hourly"
+
+    id: Mapped[str] = mapped_column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
+    source_module: Mapped[str] = mapped_column(String(64), nullable=False)
+    resource_name: Mapped[str] = mapped_column(String(255), nullable=False)
+    metric_name: Mapped[str] = mapped_column(String(128), nullable=False)
+    bucket: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)
+    value_avg: Mapped[float] = mapped_column(Float, nullable=False, default=0.0)
+    value_max: Mapped[float] = mapped_column(Float, nullable=False, default=0.0)
+    sample_count: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+
+    __table_args__ = (
+        UniqueConstraint("source_module", "resource_name", "metric_name", "bucket",
+                         name="uq_plugin_metrics_hourly_bucket"),
+        Index("ix_plugin_metrics_hourly_lookup",
+              "source_module", "resource_name", "metric_name", "bucket"),
+    )