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/signature_guard_common.go

@@ -0,0 +1,16 @@
+//go:build llm_script
+
+package main
+
+type ioDiscard struct{}
+
+func (ioDiscard) Write(p []byte) (int, error) {
+	return len(p), nil
+}
+
+func emptyIfBlank(value string) string {
+	if value == "" {
+		return "none"
+	}
+	return value
+}