Begin work on biergarten data generator pipeline

pipeline/includes/data_downloader.h

@@ -0,0 +1,26 @@
+#ifndef DATA_DOWNLOADER_H
+#define DATA_DOWNLOADER_H
+
+#include <stdexcept>
+#include <string>
+
+/// @brief Downloads and caches source geography JSON payloads.
+class DataDownloader {
+public:
+  /// @brief Initializes global curl state used by this downloader.
+  DataDownloader();
+
+  /// @brief Cleans up global curl state.
+  ~DataDownloader();
+
+  /// @brief Returns a local JSON path, downloading it when cache is missing.
+  std::string DownloadCountriesDatabase(
+      const std::string &cachePath,
+      const std::string &commit = "c5eb7772" // Stable commit: 2026-03-28 export
+  );
+
+private:
+  bool FileExists(const std::string &filePath) const;
+};
+
+#endif // DATA_DOWNLOADER_H