refactor: restructure project from bookmann to shelf

- Rename project from 'bookmann' to 'shelf'
- Move all backend/ contents to root level (flatten structure)
- Update Go module name from 'bookmann' to 'shelf'
- Update all import paths to use new 'shelf' module
- Update Dockerfile to work without backend/ subdirectory
- Update docker-compose.yml to use new structure and rename containers
- Update .gitignore for new file paths
- Update README.md with new project name and structure
- Regenerate database code with new module imports

.gitignore

@@ -197,9 +197,8 @@ coverage/
 bruno/collection.bru
 
 # Generated static assets (SvelteKit build output)
-backend/cmd/server/static/
+cmd/server/static/
 
 # Built binaries
 main
-backend/main
-backend/server
+server