Add server WriteJSON and WriteError response helpers

Eliminates repeated marshal-set-header-write boilerplate in handlers.
WriteError produces consistent {"error": "..."} JSON responses.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/respond.go

@@ -0,0 +1,29 @@
+package server
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+// WriteJSON encodes v as JSON and writes it to w with the given status code.
+// It sets Content-Type to application/json.
+func WriteJSON(w http.ResponseWriter, status int, v any) error {
+	b, err := json.Marshal(v)
+	if err != nil {
+		return err
+	}
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	_, err = w.Write(b)
+	return err
+}
+
+// WriteError writes a JSON error response with the given status code and
+// message. The response body is {"error": "<message>"}.
+func WriteError(w http.ResponseWriter, status int, msg string) error {
+	return WriteJSON(w, status, errorBody{Error: msg})
+}
+
+type errorBody struct {
+	Error string `json:"error"`
+}