fix(alerts): acknowledgements hold until resolve; clear after N OK; rule changes reset (#5059)

#5059 "Alert state machine: acknowledgement holds until resolve; N
consecutive OK to resolve; editing, disabling or deleting a rule resets it".

- The state machine is now next_step(), a pure function of one sample. An
  acknowledged alert stays acknowledged while breached; before, the next
  breaching sample re-fired it. Firing and acknowledged alerts resolve after
  consecutive_ok_required OK checks in a row (new rule column, default 1;
  rule form, rules list and MCP create/update_alert_rule clear_after_checks).
- Changing what a rule watches (source, resource, metric, operator,
  threshold) or disabling it resets its alert to inactive; a firing or
  acknowledged one records a resolved event whose new note says why, and
  nobody is notified. Web and MCP edits share alerts.service.apply_rule_change.
- Migration 0028: consecutive_ok_required, consecutive_ok_count,
  alert_events.note, and metric_value nullable (a reset has no sample).

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

steward/models/alerts.py

@@ -41,6 +41,9 @@ class AlertRule(Base):
     )
     threshold: Mapped[float] = mapped_column(Float, nullable=False)
     consecutive_failures_required: Mapped[int] = mapped_column(Integer, nullable=False, default=1)
+    # Non-breaching checks in a row before a firing (or acknowledged) alert resolves.
+    consecutive_ok_required: Mapped[int] = mapped_column(Integer, nullable=False, default=1,
+                                                         server_default="1")
     enabled: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
     # Optional admin-only action: run an Ansible playbook on transition-to-firing.
     # Shape: {source, playbook, inventory, extra_vars, limit, tags, check}. NULL = none.
@@ -62,6 +65,8 @@ class AlertState(Base):
         Enum(AlertStateEnum), nullable=False, default=AlertStateEnum.inactive
     )
     consecutive_failure_count: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    consecutive_ok_count: Mapped[int] = mapped_column(Integer, nullable=False, default=0,
+                                                      server_default="0")
     last_transition_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, default=lambda: datetime.now(timezone.utc)
     )
@@ -98,7 +103,10 @@ class AlertEvent(Base):
     )
     from_state: Mapped[str] = mapped_column(String(32), nullable=False)
     to_state: Mapped[str] = mapped_column(String(32), nullable=False)
-    metric_value: Mapped[float] = mapped_column(Float, nullable=False)
+    # None when the transition came from a change to the rule, not a sample.
+    metric_value: Mapped[float | None] = mapped_column(Float, nullable=True)
+    # Why a transition happened when the value alone doesn't say (a rule edit).
+    note: Mapped[str | None] = mapped_column(Text, nullable=True)
     transitioned_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, default=lambda: datetime.now(timezone.utc)
     )