docs: document web interface server in README

Add Web Interface section covering:
- How to build the server binary (make build-server / make build-all)
- How to run it with the --addr flag
- What the web UI does and all API endpoints (POST /api/traverse,
  GET /api/traverse/{id}, SSE stream, GET /api/health)
- Request/response shapes and SSE event format

Also update Makefile with build-server and build-all targets, and
expand Project Structure and Development sections to include the
new cmd/server/ and web/api/ packages.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: multica-agent <github@multica.ai>

Makefile

@@ -1,13 +1,19 @@
 BINARY_NAME=exploredns
+SERVER_BINARY_NAME=exploredns-server
 BUILD_DIR=bin
 GO=go
 GOFLAGS=-v
 
-.PHONY: build test lint clean cover
+.PHONY: build build-server build-all test lint clean cover
 
 build:
 	$(GO) build $(GOFLAGS) -o $(BUILD_DIR)/$(BINARY_NAME) ./cmd/exploredns
 
+build-server:
+	$(GO) build $(GOFLAGS) -o $(BUILD_DIR)/$(SERVER_BINARY_NAME) ./cmd/server
+
+build-all: build build-server
+
 test:
 	$(GO) test -v -race -coverprofile=coverage.out ./...