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/migrations/versions/0049_alert_on_a_rise.py

@@ -0,0 +1,28 @@
+"""Alert rules: a condition that fires on a rise, and the reading before this one.
+
+Revision ID: 0049_alert_on_a_rise
+Revises: 0048_tickets
+Create Date: 2026-10-30
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0049_alert_on_a_rise"
+down_revision: Union[str, None] = "0048_tickets"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # ALTER TYPE ... ADD VALUE cannot run inside a transaction block.
+    with op.get_context().autocommit_block():
+        op.execute("ALTER TYPE alertoperator ADD VALUE IF NOT EXISTS 'rose'")
+    op.add_column("alert_states", sa.Column("last_value", sa.Float, nullable=True))
+
+
+def downgrade() -> None:
+    # Postgres can't drop an enum value; 'rose' stays in the type, unused.
+    op.execute("DELETE FROM alert_rules WHERE operator = 'rose'")
+    op.drop_column("alert_states", "last_value")