fix: resolve CI lint and typecheck failures

- Fix TypeScript error in optional catch-all route by marking slug as optional
- Upgrade eslint-config-next from 15.x to 16.x for Next.js 16 compatibility
- Replace broken `next lint` command with direct `eslint` invocation
- Rewrite eslint.config.mjs to use native flat config (drop FlatCompat)
- Remove unused @eslint/eslintrc and @typescript-eslint/parser deps

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