feat(audit): add pricing signature guards and reporting

Add snapshot, signature, and drift guard support for Vertex AI, Cloudflare Workers AI, and Perplexity API, backed by a queryable audit table and recent-window view.

This commit also wires the audit query layer into daily signal materialization and report generation so structure drift becomes a first-class signal instead of a log-only artifact.

scripts/materialize_daily_signals_test.go

@@ -0,0 +1,33 @@
+//go:build llm_script
+
+package main
+
+import "testing"
+
+func TestSummarizeSignalEventTypes(t *testing.T) {
+	events := []signalModelEvent{
+		{EventType: "new_model", ModelName: "A"},
+		{EventType: "new_model", ModelName: "B"},
+		{EventType: "price_cut", ModelName: "C"},
+	}
+
+	counts := summarizeSignalEventTypes(events)
+	if counts["new_model"] != 2 {
+		t.Fatalf("new_model 计数错误: %d", counts["new_model"])
+	}
+	if counts["price_cut"] != 1 {
+		t.Fatalf("price_cut 计数错误: %d", counts["price_cut"])
+	}
+}
+
+func TestBuildSignalPageMode(t *testing.T) {
+	if got := buildSignalPageMode(signalDailySignals{}, nil); got != "calm" {
+		t.Fatalf("平静日 page_mode 错误: %q", got)
+	}
+	if got := buildSignalPageMode(signalDailySignals{NewModels: 2, PriceChanges: 1}, nil); got != "hot" {
+		t.Fatalf("高变化日 page_mode 错误: %q", got)
+	}
+	if got := buildSignalPageMode(signalDailySignals{}, []signalModelEvent{{EventType: "official_release"}}); got != "hot" {
+		t.Fatalf("官方发布日 page_mode 错误: %q", got)
+	}
+}