docs: update SPECIFICATION.md for React SPA and current endpoints
- Replace htmx with React SPA in architecture diagram and tech stack
- Rewrite Section 6 (Web Interface) with React SPA architecture,
pages table, and design patterns
- Update Section 11.1: remove old web UI routes, add /api/auth/config,
/api/uploads/presign, file attachment endpoints, thumbnail endpoint
- Fix endpoint count: 76 → 75 (actual registered routes)
- Fix migration count: 10 → 11
- Add file attachments to MVP checklist
- Update test coverage note (9 test files exist)
Closes #26
e3da072229