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-video-tasks-core/src/transport.rs

@@ -0,0 +1,106 @@
+use serde_json::Value;
+
+use crate::LocalVideoTaskStatus;
+
+pub fn parse_video_content_variant(query_string: Option<&str>) -> Option<&'static str> {
+    let mut variant = "video";
+    if let Some(query_string) = query_string {
+        for (key, value) in url::form_urlencoded::parse(query_string.as_bytes()) {
+            if key == "variant" {
+                variant = match value.as_ref() {
+                    "video" => "video",
+                    "thumbnail" => "thumbnail",
+                    "spritesheet" => "spritesheet",
+                    _ => return None,
+                };
+            }
+        }
+    }
+    Some(variant)
+}
+
+pub fn gemini_metadata_video_url(metadata: &Value) -> Option<String> {
+    metadata
+        .get("response")
+        .and_then(|value| value.get("generateVideoResponse"))
+        .and_then(|value| value.get("generatedSamples"))
+        .and_then(Value::as_array)
+        .and_then(|value| value.first())
+        .and_then(|value| value.get("video"))
+        .and_then(|value| value.get("uri"))
+        .and_then(Value::as_str)
+        .map(str::to_string)
+}
+
+pub fn map_openai_task_status(status: LocalVideoTaskStatus) -> &'static str {
+    match status {
+        LocalVideoTaskStatus::Submitted | LocalVideoTaskStatus::Queued => "queued",
+        LocalVideoTaskStatus::Processing => "processing",
+        LocalVideoTaskStatus::Completed => "completed",
+        LocalVideoTaskStatus::Failed
+        | LocalVideoTaskStatus::Cancelled
+        | LocalVideoTaskStatus::Expired => "failed",
+        LocalVideoTaskStatus::Deleted => "deleted",
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use serde_json::json;
+
+    use crate::LocalVideoTaskStatus;
+
+    use super::{gemini_metadata_video_url, map_openai_task_status, parse_video_content_variant};
+
+    #[test]
+    fn parses_supported_video_content_variants() {
+        assert_eq!(parse_video_content_variant(None), Some("video"));
+        assert_eq!(
+            parse_video_content_variant(Some("variant=thumbnail")),
+            Some("thumbnail")
+        );
+        assert_eq!(
+            parse_video_content_variant(Some("variant=spritesheet")),
+            Some("spritesheet")
+        );
+        assert_eq!(parse_video_content_variant(Some("variant=invalid")), None);
+    }
+
+    #[test]
+    fn extracts_gemini_metadata_video_url() {
+        let metadata = json!({
+            "response": {
+                "generateVideoResponse": {
+                    "generatedSamples": [
+                        {
+                            "video": {
+                                "uri": "https://example.com/video.mp4"
+                            }
+                        }
+                    ]
+                }
+            }
+        });
+
+        assert_eq!(
+            gemini_metadata_video_url(&metadata).as_deref(),
+            Some("https://example.com/video.mp4")
+        );
+    }
+
+    #[test]
+    fn maps_openai_task_status() {
+        assert_eq!(
+            map_openai_task_status(LocalVideoTaskStatus::Queued),
+            "queued"
+        );
+        assert_eq!(
+            map_openai_task_status(LocalVideoTaskStatus::Completed),
+            "completed"
+        );
+        assert_eq!(
+            map_openai_task_status(LocalVideoTaskStatus::Failed),
+            "failed"
+        );
+    }
+}