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

static/placeholder-book.svg

@@ -0,0 +1,9 @@
+<svg width="200" height="300" xmlns="http://www.w3.org/2000/svg">
+  <rect width="200" height="300" fill="#f3f4f6"/>
+  <rect x="20" y="20" width="160" height="260" fill="#ffffff" stroke="#e5e7eb" stroke-width="2"/>
+  <text x="100" y="120" text-anchor="middle" font-family="Arial, sans-serif" font-size="14" fill="#6b7280">Book Cover</text>
+  <line x1="40" y1="160" x2="160" y2="160" stroke="#e5e7eb" stroke-width="1"/>
+  <line x1="40" y1="180" x2="160" y2="180" stroke="#e5e7eb" stroke-width="1"/>
+  <line x1="40" y1="200" x2="160" y2="200" stroke="#e5e7eb" stroke-width="1"/>
+  <line x1="40" y1="220" x2="160" y2="220" stroke="#e5e7eb" stroke-width="1"/>
+</svg>