refactor: 移除 Python upstream 依赖，清理全部 legacy/Python 兼容层

- 移除 upstream_base_url 参数及 AETHER_GATEWAY_UPSTREAM 环境变量，gateway 不再需要指向 Python 宿主
- 删除所有 LEGACY_*/PYTHON_* 常量、路由组、header 定义及 sunset/phaseout 机制
- 将 legacy_gateway_bridge 重命名为 internal_gateway，executor 相关命名统一为 execution_runtime
- dev.sh 新增 Postgres/Redis 预检查，移除 upstream 相关启动参数和提示
- 新增 ai_public 路由处理器
- 全量适配 handler、test、state、control 等模块的命名和接口变更

apps/aether-gateway/src/handlers/public/mod.rs

@@ -1,9 +1,40 @@
-pub(crate) use super::*;
-
+mod ai_public;
 mod catalog_helpers;
 mod support;
 mod system_modules_helpers;
 
-pub(crate) use self::catalog_helpers::*;
-pub(crate) use self::support::*;
-pub(crate) use self::system_modules_helpers::*;
+pub(crate) use self::ai_public::{
+    ai_public_local_requires_buffered_body, maybe_build_local_ai_public_response,
+};
+pub(crate) use self::catalog_helpers::{
+    admin_requested_force_stream, api_format_display_name, build_api_format_health_monitor_payload,
+    build_public_catalog_models_payload, build_public_catalog_search_models_payload,
+    build_public_health_timeline, build_public_providers_payload, normalize_admin_base_url,
+    provider_key_api_formats, request_candidate_event_unix_secs, request_candidate_status_label,
+    ApiFormatHealthMonitorOptions,
+};
+pub(crate) use self::system_modules_helpers::{
+    admin_module_by_name, admin_module_name_from_enabled_path, admin_module_name_from_status_path,
+    apply_admin_email_template_update, apply_admin_system_config_update,
+    apply_admin_system_settings_update, build_admin_api_formats_payload,
+    build_admin_email_template_payload, build_admin_email_templates_payload,
+    build_admin_keys_grouped_by_format_payload, build_admin_module_runtime_state,
+    build_admin_module_status_payload, build_admin_module_validation_result,
+    build_admin_modules_status_payload, build_admin_system_check_update_payload,
+    build_admin_system_config_detail_payload, build_admin_system_config_export_payload,
+    build_admin_system_configs_payload, build_admin_system_settings_payload,
+    build_admin_system_stats_payload, build_admin_system_users_export_payload,
+    build_public_auth_modules_status_payload, capability_detail_by_name, current_aether_version,
+    delete_admin_system_config, enabled_key_capability_short_names,
+    escape_admin_email_template_html, ldap_module_config_is_valid, module_available_from_env,
+    preview_admin_email_template, read_admin_email_template_payload,
+    render_admin_email_template_html, reset_admin_email_template,
+    serialize_admin_system_users_export_wallet, serialize_public_capability,
+    supported_capability_names, system_config_bool, system_config_string,
+    AdminSetModuleEnabledRequest, PUBLIC_CAPABILITY_DEFINITIONS,
+};
+
+pub(crate) use self::support::{
+    build_unhandled_public_support_response, matches_model_mapping_for_models,
+    maybe_build_local_admin_announcements_response, maybe_build_local_public_support_response,
+};