fix(anomalies): a burst within what a metric reaches in most hours opens nothing (#6727)

#6727 "A metric that bursts now and then opens anomalies at hours whose usual
peak reads 0". usual_peak is the p90 of a slot's hourly max, so a metric that
bursts in under about 1 hour in 10 reads 0 there however often it bursts across
the day. #6723 "An anomaly opens on a burst of minutes inside the metric's usual
peak, so most rows aren't outside the norm" therefore still opened on those.

usual_anywhere is the median, across the hours of the day, of each hour's
highest peak. Height is now judged against the greater of it and the slot's
usual peak. Bursts at any hour lift the median; a job at a time of its own
raises a few hours and leaves it low, so the same level at another time of day
is still an anomaly, as System 182 rules. A held level still opens.

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

steward/models/norms.py

@@ -24,6 +24,8 @@ class MetricNorm(Base):
     # peaked: the 90th percentile, across past days, of each hour's p95 and max.
     usual_hold: Mapped[float] = mapped_column(Float, nullable=False)
     usual_peak: Mapped[float] = mapped_column(Float, nullable=False)
+    # How high the metric has gone in most hours of the day, whatever the slot.
+    usual_anywhere: Mapped[float] = mapped_column(Float, nullable=False, default=0.0, server_default="0")
     days_seen: Mapped[int] = mapped_column(Integer, nullable=False)
     updated_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)