fix: resolve lint errors from security changes

- Remove unused `/* global process */` in next.config.mjs
- Attach cause to rethrown error in secret.ts legacy key expiry

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

next.config.mjs

@@ -1,5 +1,3 @@
-/* global process */
-
 // When building under Node.js (not Bun), redirect bun:sqlite to a better-sqlite3 shim
 // so `next build` works locally without Bun installed.
 const isBun = typeof globalThis.Bun !== 'undefined';