docs: add OpenAPI documentation with Scalar UI

- Add swaggo annotations to all HTTP handlers
- Add Swagger/OpenAPI spec generation with swag
- Create separate docs server binary (drexa-docs)
- Add Makefile with build, run, and docs targets
- Configure Scalar as the API documentation UI

Run 'make docs' to regenerate, 'make run-docs' to serve.

apps/backend/Makefile

@@ -0,0 +1,41 @@
+.PHONY: build build-docs build-all run run-docs docs install-tools fmt clean
+
+# Build the API server
+build:
+	go build -o bin/drexa ./cmd/drexa
+
+# Build the documentation server
+build-docs:
+	go build -o bin/drexa-docs ./cmd/docs
+
+# Build all binaries
+build-all: build build-docs
+
+# Run the API server
+run:
+	go run ./cmd/drexa --config config.yaml
+
+# Run the documentation server
+run-docs:
+	go run ./cmd/docs --port 8081 --api-url http://localhost:8080
+
+# Generate API documentation
+docs:
+	@echo "Generating OpenAPI documentation..."
+	swag init -g cmd/drexa/main.go -o docs --parseDependency --parseInternal --outputTypes go,json,yaml
+	@echo "Documentation generated in docs/"
+	@echo "Run 'make run-docs' to start the documentation server"
+
+# Install development tools
+install-tools:
+	go install github.com/swaggo/swag/cmd/swag@latest
+
+# Format and lint
+fmt:
+	go fmt ./...
+	swag fmt
+
+# Clean build artifacts
+clean:
+	rm -rf bin/
+	rm -f docs/swagger.json docs/swagger.yaml