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/003_add_ebook_metadata_fields.up.sql

@@ -0,0 +1,8 @@
+-- Add additional metadata fields to ebooks table
+ALTER TABLE ebooks ADD COLUMN series VARCHAR(255);
+ALTER TABLE ebooks ADD COLUMN series_number INTEGER;
+ALTER TABLE ebooks ADD COLUMN tags TEXT;
+ALTER TABLE ebooks ADD COLUMN asin VARCHAR(20);
+ALTER TABLE ebooks ADD COLUMN date_published DATE;
+ALTER TABLE ebooks ADD COLUMN publisher VARCHAR(255);
+ALTER TABLE ebooks ADD COLUMN contributors TEXT;