feat: remove legacy alert rules, superseded by automations

Alert rules already auto-migrate into automations on boot and the old
engine stands down after. Deletes the rule/firing tables, routes, UI,
and AI tools; keeps notification channels since automations still use
them. Also fixes automations' notify step, which still called the
deleted repository.

src/ui/tests/api/alerts-api.test.ts

@@ -1,45 +0,0 @@
-import { describe, expect, it } from "vitest";
-import { mapAlertFiring } from "../../api/alerts-api";
-
-describe("mapAlertFiring", () => {
-  it("normalizes sqlite snake_case rows for the alerts UI", () => {
-    expect(
-      mapAlertFiring({
-        id: 9,
-        user_id: "u1",
-        rule_id: 4,
-        host_id: 12,
-        host_name: "db-01",
-        fired_at: "2026-06-30 12:00:00",
-        resolved_at: null,
-        value: 91,
-        message: "CPU threshold exceeded",
-        severity: "critical",
-        acknowledged: 0,
-        rule_name: "CPU",
-      }),
-    ).toEqual({
-      id: 9,
-      userId: "u1",
-      ruleId: 4,
-      hostId: 12,
-      hostName: "db-01",
-      firedAt: "2026-06-30 12:00:00",
-      resolvedAt: null,
-      value: 91,
-      message: "CPU threshold exceeded",
-      severity: "critical",
-      acknowledged: false,
-      ruleName: "CPU",
-    });
-  });
-
-  it("uses safe defaults for malformed rows", () => {
-    const firing = mapAlertFiring({ acknowledged: 1, severity: "bad" });
-
-    expect(firing.id).toBe(0);
-    expect(firing.hostName).toBe("");
-    expect(firing.severity).toBe("warning");
-    expect(firing.acknowledged).toBe(true);
-  });
-});