Fix MCP /mcp-protocol/ 421 error: disable DNS rebinding protection and fix SPA fallback ...

- Disable MCP SDK transport_security DNS rebinding protection (nginx handles it)
- Fix SPA fallback to exclude /mcp-protocol without trailing slash
- Add transport_security import to mcp_protocol.py

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Production-ready rewrite: UI kit integration, admin backups, sorting, profile editing, and security hardening ...

- Frontend: complete rewrite using gnexus-ui-kit components (GnNavigationShell,
  GnTable, GnTimeline, GnModal, GnToastProvider, GnSkeleton, etc.)
- Frontend: browser route sync (/secrets/:id, /audit/:id), pagination,
  real-time search debounce, skeleton loading states, retry with backoff
- Frontend: profile editing modal, token creation with scope selection,
  import/export, settings metrics, admin users table
- Backend: backup/restore endpoints for PostgreSQL and SQLite via pg_dump/psql
- Backend: admin backup CRUD, file download endpoint
- Backend: stats endpoint, PATCH /me profile endpoint, CORS middleware,
  graceful shutdown logging
- Backend: table sorting (sort_by/sort_dir) in list_secrets
- Backend: OAuth return_to validation, rate limiting on sensitive endpoints
- MCP: validation fixes, tool description improvements, protocol cleanup
- Tests: 44 passing including new backup test suite
- Docker: add postgresql-client, backups volume, deployment docs
- Docs: backup/restore instructions in deployment guide

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Add official MCP transport and production hardening

Improve UI layout and serve kit fonts

Serve frontend build from FastAPI

Implement initial gnexus-creds MVP scaffold