feat(norms): Steward learns each metric's normal per hour of day, then week (#5739)

A metric registered with learn_normal() gets, for every resource reporting
it, a usual hold (the 90th percentile across past days of each hour's p95)
and a usual peak for each slot:
- an hour of the day once 5 days of history exist;
- an hour of the week once history reaches back 3 weeks, so weekly jobs are
  learned too.
Each slot takes in the hours either side, so a job that runs a little late
stays normal. One SQL statement relearns everything hourly into
metric_norms, from the rollup plus the raw window not rolled up yet;
norms_at() reads the slot a moment falls in. Nothing is registered yet:
Traefik (#5741) and hosts (#5742) opt in.

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

steward/models/norms.py

@@ -0,0 +1,27 @@
+from __future__ import annotations
+from datetime import datetime
+from sqlalchemy import DateTime, Float, Integer, SmallInteger, String
+from sqlalchemy.orm import Mapped, mapped_column
+from .base import Base
+
+
+class MetricNorm(Base):
+    """What one metric usually does in one time slot (#5739), learned from
+    its own history by steward.core.norms.rebuild_norms.
+
+    `kind` says what a slot is: "day" (0–23, the UTC hour of day) or "week"
+    (0–167, Monday 00:00 UTC first). A key has slots of one kind at a time.
+    The whole table is rebuilt each pass, so a row is never edited."""
+    __tablename__ = "metric_norms"
+
+    source_module: Mapped[str] = mapped_column(String(64), primary_key=True)
+    resource_name: Mapped[str] = mapped_column(String(255), primary_key=True)
+    metric_name: Mapped[str] = mapped_column(String(128), primary_key=True)
+    slot: Mapped[int] = mapped_column(SmallInteger, primary_key=True)
+    kind: Mapped[str] = mapped_column(String(8), nullable=False)
+    # The level the slot's hours usually held, and how high they usually
+    # 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)
+    days_seen: Mapped[int] = mapped_column(Integer, nullable=False)
+    updated_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)