feat: production release with React SPA, file attachments, and deploy tooling

Backend:
- Add file_handlers.go: presigned upload/download for item attachments
- Add item_files.go: item file and thumbnail DB operations
- Add migration 011: item_files table and thumbnail_key column
- Update items/projects/relationships DB with extended field support
- Update routes: React SPA serving from web/dist, file upload endpoints
- Update auth handlers and middleware for cookie + bearer token auth
- Remove Go HTML templates (replaced by React SPA)
- Update storage client for presigned URL generation

Frontend:
- Add TagInput component for tag/keyword entry
- Add SVG assets for Silo branding and UI icons
- Update API client and types for file uploads, auth, extended fields
- Update AuthContext for session-based auth flow
- Update LoginPage, ProjectsPage, SchemasPage, SettingsPage
- Fix tsconfig.node.json

Deployment:
- Update config.prod.yaml: single-binary SPA layout at /opt/silo
- Update silod.service: ReadOnlyPaths for /opt/silo
- Add scripts/deploy.sh: build, package, ship, migrate, start
- Update docker-compose.yaml and Dockerfile
- Add frontend-spec.md design document

web/assets/silo-auth.svg

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