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

.dockerignore

@@ -0,0 +1,10 @@
+.git
+.gitignore
+README.md
+*.md
+.env
+.DS_Store
+.vscode
+.idea
+tmp/
+logs/