Pipeline: add CURL/WebClient & Wikipedia service

Introduce a pluggable web client interface and concrete CURL implementation: adds IWebClient, CURLWebClient, and CurlGlobalState (headers + curl_web_client.cpp). DataDownloader now accepts an IWebClient and delegates downloads. Add WikipediaService for cached Wikipedia summary lookups. Refactor SqliteDatabase to return full City records and update consumers accordingly. Improve JsonLoader to use batched transactions during streaming parses. Enhance LlamaGenerator with sampling options, increased token limits, JSON extraction/validation, and other parsing helpers. Modernize CMake: set policy/version, add project_options, simplify FetchContent usage (spdlog), require Boost components (program_options/json), list pipeline sources explicitly, and tweak post-build/memcheck targets. Update README to match implementation changes and new CLI/config conventions.

pipeline/includes/web_client.h

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <string>
+
+class IWebClient {
+public:
+  virtual ~IWebClient() = default;
+
+  // Downloads content from a URL to a file. Throws on error.
+  virtual void DownloadToFile(const std::string &url,
+                              const std::string &filePath) = 0;
+
+  // Performs a GET request and returns the response body as a string. Throws on
+  // error.
+  virtual std::string Get(const std::string &url) = 0;
+
+  // URL-encodes a string.
+  virtual std::string UrlEncode(const std::string &value) = 0;
+};