Backend review fixes (stages B1/B2): salted master KDF, AAD-bound ciphertexts, hardened destructive endpoints ...

- crypto: master-key derivation is now scrypt (n=2^15) with a per-install
  salt (GNEXUS_CREDS_MASTER_KEY_SALT); plain sha256 kept as the legacy
  path so pre-salt data still decrypts; salt is mandatory in production
- crypto: encrypted field envelopes are now bound by aad to
  "<user_id>:<secret_id>:<version_id>" (pre-generated version ids) so a
  ciphertext transplanted between rows of the same user fails auth
  instead of silently decrypting; unbound legacy envelopes decrypt
  through a single fallback and are re-bound by scripts/migrate-crypto.py
- secrets.notes widened 140 -> 255 (model, schemas, alembic 0002)
- list_secrets: sort_by restricted to a column whitelist (422 otherwise);
  limit clamp aligned with the API layer (1..200)
- update_secret: tag normalization moved to schemas.normalize_tags; the
  metadata audit now records the tags diff BEFORE the rows change (the
  old value used to be read after the rows were cleared)
- DELETE /account-data now requires Scope.admin (ui channel unaffected)
  plus the sensitive rate limiter, and audits the deleted count
- POST /import caps payload.secrets at 1000
- POST /admin/restore takes a typed payload and resolves the filename
  against the backup listing; restore_backup itself refuses files
  outside the backup dir
- production settings: master_key_salt required, wildcard cors_origins
  rejected
- tests: 62 -> 78 (crypto KDF/aad, sort whitelist, notes cap, import cap,
  account-data scopes, restore validation, config prod rules)

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

Adopt shield keyhole logo, serve dist public assets from SPA fallback ...

gnexus-creds now has its own mark: a shield with a keyhole, drawn in the
gnexus-ui-kit language (dark tile, line-art strokes, square keyhole
terminal in the role color). Ship it as frontend/public/logo.svg, use it
in the navigation shell instead of the external gnexus.space PNG, and
register it as the SVG favicon.

The SPA fallback in main.py returned index.html for every unknown path,
so /logo.svg rendered the HTML shell instead of the image; serve real
files that exist in frontend/dist first and fall back only afterwards.
Also drop two unused imports flagged by ruff.

Co-Authored-By: Claude Code <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>