refactor: 拆分 gateway 单体为独立 crate，新增 systemd 部署方案

将 gateway 内部的 model-fetch、provider-transport、scheduler-core、
usage-runtime、video-tasks-core 模块提取为独立 crate；重构 gateway
内部模块结构（state/router/cache/data/query 等）；移除大量遗留模块
文件；新增 systemd 二进制部署骨架及相关文档；更新前端 usage 相关
API 和组件。

crates/aether-provider-transport/src/kiro/url.rs

@@ -0,0 +1,71 @@
+use super::super::url::build_passthrough_path_url;
+use super::credentials::DEFAULT_REGION;
+
+pub const GENERATE_ASSISTANT_RESPONSE_PATH: &str = "/generateAssistantResponse";
+pub const KIRO_ENVELOPE_NAME: &str = "kiro:generateAssistantResponse";
+
+pub fn resolve_kiro_base_url(upstream_base_url: &str, api_region: Option<&str>) -> String {
+    let region = api_region
+        .map(str::trim)
+        .filter(|value| !value.is_empty())
+        .unwrap_or(DEFAULT_REGION);
+    upstream_base_url
+        .trim()
+        .replace("{region}", region)
+        .trim_end_matches('/')
+        .to_string()
+}
+
+pub fn build_kiro_generate_assistant_response_url(
+    upstream_base_url: &str,
+    query: Option<&str>,
+    api_region: Option<&str>,
+) -> Option<String> {
+    let upstream_base_url = resolve_kiro_base_url(upstream_base_url, api_region);
+    build_passthrough_path_url(
+        upstream_base_url.as_str(),
+        GENERATE_ASSISTANT_RESPONSE_PATH,
+        query,
+        &[],
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{
+        build_kiro_generate_assistant_response_url, resolve_kiro_base_url,
+        GENERATE_ASSISTANT_RESPONSE_PATH, KIRO_ENVELOPE_NAME,
+    };
+
+    #[test]
+    fn exposes_kiro_request_constants() {
+        assert_eq!(
+            GENERATE_ASSISTANT_RESPONSE_PATH,
+            "/generateAssistantResponse"
+        );
+        assert_eq!(KIRO_ENVELOPE_NAME, "kiro:generateAssistantResponse");
+    }
+
+    #[test]
+    fn builds_generate_assistant_response_url() {
+        assert_eq!(
+            build_kiro_generate_assistant_response_url(
+                "https://kiro.{region}.example?tenant=demo",
+                Some("stream=true"),
+                Some("us-west-2")
+            )
+            .as_deref(),
+            Some(
+                "https://kiro.us-west-2.example/generateAssistantResponse?stream=true&tenant=demo"
+            )
+        );
+    }
+
+    #[test]
+    fn resolves_region_placeholder_in_base_url() {
+        assert_eq!(
+            resolve_kiro_base_url("https://kiro.{region}.example/", Some("us-west-2")),
+            "https://kiro.us-west-2.example"
+        );
+    }
+}