chore: add .gitignore with data-backup exclusion

- Exclude backend/src/data-backup/ from version control
- Add common ignores: node_modules, .env, IDE files, logs

.gitignore

@@ -0,0 +1,25 @@
+# Development data backups
+backend/src/data-backup/
+
+# Node modules
+node_modules/
+
+# Environment files
+.env
+.env.local
+
+# IDE
+.vscode/
+.idea/
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Logs
+*.log
+npm-debug.log*
+
+# Build outputs
+dist/
+build/