Add secrets infrastructure: DB tables, shared types, env binding model, and migration improvements

Introduce company_secrets and company_secret_versions tables for
encrypted secret storage. Add EnvBinding discriminated union (plain vs
secret_ref) to replace raw string env values in adapter configs. Add
hiddenAt column to issues for soft-hiding. Improve migration system
with journal-ordered application and manual fallback when Drizzle
migrator can't reconcile history.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/db/src/schema/index.ts

@@ -16,3 +16,5 @@ export { costEvents } from "./cost_events.js";
 export { approvals } from "./approvals.js";
 export { approvalComments } from "./approval_comments.js";
 export { activityLog } from "./activity_log.js";
+export { companySecrets } from "./company_secrets.js";
+export { companySecretVersions } from "./company_secret_versions.js";