feat(alerts): a rule can fire when a reading rises, "rose by N" (#6724)

A rule compared a reading with a fixed number, so Synology Disk 1's bad
sectors going from 3,160 to 3,392 could say nothing: a count that only grows
is soon past any line. "rose by" compares each reading with the one before it,
kept on the alert's state and only for such a rule. A fall (a replaced disk)
doesn't fire. On the rule form, through MCP, and worded in emails, the alert
sentence, history and the timeline.

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

steward/alerts/routes.py

@@ -15,7 +15,7 @@ from steward.models.users import UserRole
 
 alerts_bp = Blueprint("alerts", __name__, url_prefix="/alerts")
 
-_OPERATORS = [(op.value, op.value) for op in AlertOperator]
+_OPERATORS = [(op.value, op.said) for op in AlertOperator]
 
 def _form_condition(form) -> dict:
     """The operator and threshold the form asks for. An up/down metric's form