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

sqlc.yaml

@@ -0,0 +1,17 @@
+version: "2"
+sql:
+  - engine: "postgresql"
+    schema: "migrations"
+    queries: "internal/database/queries"
+    gen:
+      go:
+        package: "database"
+        out: "internal/database"
+        sql_package: "pgx/v5"
+        emit_db_tags: true
+        emit_prepared_queries: true
+        emit_interface: true
+        emit_exact_table_names: true
+        emit_empty_slices: true
+        emit_exported_queries: true
+        emit_json_tags: true