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.
Add plan catalog and subscription schema support, seed baselines, and real importers for core domestic subscriptions plus stable official pricing sources.
This commit also hardens the shared fetch layers so the importers can support live collection and database writes instead of relying on manual placeholders alone.
