chore(db): create database backup in backend/db/backup and add backup-db Makefile command

- Save SQLite binary backup (anl.db) and complete SQL dump (anl_dump.sql) in backend/db/backup/
- Add make backup-db target in backend/Makefile for easy one-command backups
- Allow tracking of anl_dump.sql in backend/.gitignore

backend/Makefile

@@ -37,5 +37,11 @@ vet:
 test: fmt-check vet
 	go test -v ./...
 
+backup-db:
+	@mkdir -p db/backup
+	sqlite3 $(DB_PATH) ".backup 'db/backup/anl.db'"
+	sqlite3 $(DB_PATH) ".dump" > db/backup/anl_dump.sql
+	@echo "✅ Database backed up to db/backup/anl.db and db/backup/anl_dump.sql"
+
 clean:
 	rm -rf $(BIN_DIR) *.db*