feat(gateway): migrate configuration to confs service (v0.7.0)

Migrate gateway from local -i ini file to the unified confs configuration
service, following the same two-layer pattern as Python services (dnfs/db/static):

- Add confs/configs/gateway.ini with all existing settings plus new
  [Downstream] section for db/captcha/smtp host:port (previously hardcoded).
- Conf.hpp: add fromString() to parse ini from text (confs response),
  env()/env_int() static helpers for environment variables, public default
  constructor, and empty-filename guard in file constructor.
- DbC.hpp: add init(host, port) static method and static _host/_port members,
  replacing hardcoded "127.0.0.1":8005. Defaults preserved for backward compat.
- main.cpp:
  * Rewrite config loading: bootstrap CONFS_HOST/CONFS_PORT from env
    (default 127.0.0.1:8010), fetch gateway.ini via pymsg from confs
    (retry_on_refused=true), parse with ConfIni::fromString().
  * -i flag is now optional; when provided, reads local ini (dev/debug mode).
  * Add downstream host/port globals (_DB_HOST/_DB_PORT, _CAPTCHA_HOST/_CAPTCHA_PORT,
    _SMTP_HOST/_SMTP_PORT, _CONFS_HOST/_CONFS_PORT), populated from [Downstream].
  * Environment variable overrides (highest priority): LISTEN_ADDR/LISTEN_PORT,
    DB_HOST/DB_PORT, CAPTCHA_HOST/CAPTCHA_PORT, SMTP_HOST/SMTP_PORT.
  * Config integrity validation: required fields (Server.address/port,
    Proxy.default, Downstream.db_host/db_port) cause startup failure if missing.
  * Initialize DbC::init() and logs().set_endpoint() after config load.
  * Replace all hardcoded "127.0.0.1":8005/8008/8009 in captcha/smtp/db calls
    with configurable globals.
  * Add downstream service info to startup log output.
  * Fix: -v/--version flag now actually prints version and exits.
  * Bump version to 0.7.0.
- _run.sh: remove -i example.ini default; now starts without args (confs mode),
  passes "$@" through for optional -i override.
- Update README with new config loading docs, Downstream config section,
  updated directory structure, and v0.7.0 changelog.

gateway/inc/Conf.hpp

@@ -1,6 +1,9 @@
 #pragma once
 #include <iostream>
 #include <exception>
+#include <string>
+#include <sstream>
+#include <cstdlib>
 #include <boost/property_tree/ptree.hpp>
 #include <boost/property_tree/ini_parser.hpp>
 namespace property_tree = boost::property_tree;
@@ -9,22 +12,54 @@ namespace qing {
 
 	class ConfIni {
 	public:
+		/// 默认构造（空配置，后续通过赋值覆盖，仅供 confs 加载流程内部使用）
+		ConfIni() = default;
+
+		/// 从文件路径解析 ini（保留用于开发/调试模式）
 		ConfIni(const std::string &filename) {
+			if (filename.empty()) return;  // 空文件名不报错，后续会通过赋值覆盖
 			try {
-				std::cout << "open and parse ini file: " << filename << std::endl;
+				std::cout << "[conf] 从文件加载配置: " << filename << std::endl;
 				property_tree::ini_parser::read_ini(filename, pt);
 			}
 			catch (const boost::property_tree::ini_parser_error& e) {
-				std::cerr << "INI parsing error: " << e.what() << std::endl;
+				std::cerr << "[conf] INI 解析错误: " << e.what() << std::endl;
 				throw IniConfigureFileParseException("INI parsing error!");
 			}
 		}
 
+		/// 从 ini 文本字符串解析（用于从 confs 拉取后解析）
+		static ConfIni fromString(const std::string &ini_text) {
+			ConfIni c;
+			try {
+				std::istringstream iss(ini_text);
+				property_tree::ini_parser::read_ini(iss, c.pt);
+			}
+			catch (const boost::property_tree::ini_parser_error& e) {
+				std::cerr << "[conf] INI 文本解析错误: " << e.what() << std::endl;
+				throw IniConfigureFileParseException("INI parsing error!");
+			}
+			return c;
+		}
+
 		template <typename T>
 		T get(const std::string &path, const T &_defaut) {
 			return pt.get<T>(path, _defaut);
 		}
 
+		/// 读取环境变量，不存在则返回默认值
+		static std::string env(const char *name, const std::string &default_val = "") {
+			const char *v = std::getenv(name);
+			return v ? std::string(v) : default_val;
+		}
+
+		static int env_int(const char *name, int default_val) {
+			const char *v = std::getenv(name);
+			if (!v || !*v) return default_val;
+			try { return std::stoi(v); }
+			catch (...) { return default_val; }
+		}
+
 		class IniConfigureFileParseException: public std::exception {
 		public:
 			explicit IniConfigureFileParseException(const std::string &msg)