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

migrations/005_create_user_ebook_folders.up.sql

@@ -0,0 +1,11 @@
+-- Create user_ebook_folders table for multiple folders per user
+CREATE TABLE user_ebook_folders (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    folder_path VARCHAR(500) NOT NULL,
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    UNIQUE(user_id, folder_path)
+);
+
+-- Create index for faster lookups
+CREATE INDEX idx_user_ebook_folders_user_id ON user_ebook_folders(user_id);