feat(metrics): each rolled-up hour keeps the level it held, its p95 (#5738)

plugin_metrics_hourly gains value_p95 beside value_avg and value_max. A
two-minute backup spike raises the hour's peak and leaves its p95 low; an
hour spent at that level raises both. Learned normals (milestone 513) need
that difference to call a recurring spike usual without calling the same
level held at another hour usual too. Hours rolled up before this keep a
NULL p95 and read as their average.

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

steward/migrations/versions/0044_hourly_p95.py

@@ -0,0 +1,26 @@
+"""Each hour of the metric rollup keeps the level it held, its 95th percentile,
+beside its average and peak (#5738), so a learned normal can tell a brief
+spike (a high peak, a low p95) from a level the metric stayed at.
+
+Revision ID: 0044_hourly_p95
+Revises: 0043_outbox_alert_words
+Create Date: 2026-10-14
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0044_hourly_p95"
+down_revision: Union[str, None] = "0043_outbox_alert_words"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Hours rolled up before this have no p95 to recover (their raw samples
+    # are gone), so they stay NULL and read as their average.
+    op.add_column("plugin_metrics_hourly", sa.Column("value_p95", sa.Float, nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("plugin_metrics_hourly", "value_p95")