feat(api): GET /api/tracks/{id} handler with parent metadata

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/api/errors.go

@@ -23,3 +23,12 @@ func writeErr(w http.ResponseWriter, status int, code, message string) {
 	w.WriteHeader(status)
 	_ = json.NewEncoder(w).Encode(errorBody{Error: errorPayload{Code: code, Message: message}})
 }
+
+// writeJSON emits a 2xx response with the given body JSON-encoded. Mirrors
+// writeErr's shape so handlers have one shape for success and one for
+// failure.
+func writeJSON(w http.ResponseWriter, status int, body any) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(body)
+}