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

internal/database/connection.go

@@ -0,0 +1,20 @@
+package database
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+func NewConnection(databaseURL string) (*pgxpool.Pool, error) {
+	pool, err := pgxpool.New(context.Background(), databaseURL)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := pool.Ping(context.Background()); err != nil {
+		return nil, err
+	}
+
+	return pool, nil
+}