Drop the legacy cache tables after the production purge

apps/dashboard/drizzle/0007_drop_legacy_cache_tables.sql

@@ -0,0 +1,16 @@
+-- Drop the cache-era tables removed from the schema in #193 (see
+-- docs/platform-plan.md, "Incidents"). Only apply after the throttled
+-- production purge has emptied them: dropping a multi-GB table in one
+-- statement blocks the deploy pipeline. Check first with
+--   wrangler d1 execute <db> --remote --command "SELECT
+--     (SELECT COUNT(*) FROM github_webhook_event) AS webhook_events,
+--     (SELECT COUNT(*) FROM github_revalidation_signal) AS signals,
+--     (SELECT COUNT(*) FROM github_cache_namespace) AS namespaces,
+--     (SELECT COUNT(*) FROM github_response_cache) AS responses"
+DROP TABLE IF EXISTS `github_webhook_event`;
+--> statement-breakpoint
+DROP TABLE IF EXISTS `github_revalidation_signal`;
+--> statement-breakpoint
+DROP TABLE IF EXISTS `github_cache_namespace`;
+--> statement-breakpoint
+DROP TABLE IF EXISTS `github_response_cache`;