feat: add admin handler (modpack CRUD, file upload, manifest, launcher release)
- modpack CRUD: GET/POST/PUT/DELETE /api/admin/modpacks
- file upload: POST /api/admin/modpacks/{slug}/upload — multipart, ZIP extraction, CAS storage
- manifest: POST /api/admin/modpacks/{slug}/manifest — scan instance dir, generate manifest.json
- launcher release: POST /api/admin/launcher/release — CI/CD endpoint, SHA-256 verify, DB registration
- auth middleware: Bearer token + admin role check + X-CI-Token for CI/CD
- zip-slip protection in file extraction
Co-Authored-By: OWL <noreply@anthropic.com>