chore: add .gitignore to backend directory

Exclude build artifacts and runtime data from version control:
- server binary
- data directory
- Go-related build artifacts
- IDE-specific files

backend/.gitignore

@@ -0,0 +1,34 @@
+# Binaries
+server
+
+# Data directory
+data/
+
+# Go
+*.exe
+*.exe~
+*.dll
+*.so
+*.dylib
+go.work
+go.work.sum
+
+# Test binary, built with `go test -c`
+*.test
+
+# Output of the go coverage tool, specifically when used with LiteIDE
+*.out
+
+# Dependency directories
+vendor/
+
+# Go workspace file
+go.work
+go.work.sum
+
+# IDE
+.vscode/
+.idea/
+*.swp
+*.swo
+*~