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/read_side.rs

@@ -0,0 +1,66 @@
+use aether_data::repository::video_tasks::{StoredVideoTask, VideoTaskLookupKey};
+use aether_data::DataLayerError;
+use async_trait::async_trait;
+
+use crate::{
+    map_gemini_stored_task_to_read_response, map_openai_stored_task_to_read_response,
+    resolve_video_task_read_lookup_key, LocalVideoTaskReadResponse,
+};
+
+#[async_trait]
+pub trait StoredVideoTaskReadSide: Send + Sync {
+    async fn find_stored_video_task(
+        &self,
+        key: VideoTaskLookupKey<'_>,
+    ) -> Result<Option<StoredVideoTask>, DataLayerError>;
+}
+
+pub async fn read_data_backed_video_task_response(
+    state: &impl StoredVideoTaskReadSide,
+    route_family: Option<&str>,
+    request_path: &str,
+) -> Result<Option<LocalVideoTaskReadResponse>, DataLayerError> {
+    match route_family {
+        Some("openai") => read_openai_video_task_response(state, request_path).await,
+        Some("gemini") => read_gemini_video_task_response(state, request_path).await,
+        _ => Ok(None),
+    }
+}
+
+async fn read_openai_video_task_response(
+    state: &impl StoredVideoTaskReadSide,
+    request_path: &str,
+) -> Result<Option<LocalVideoTaskReadResponse>, DataLayerError> {
+    let Some(lookup) = resolve_video_task_read_lookup_key(Some("openai"), request_path) else {
+        return Ok(None);
+    };
+
+    let Some(task) = state.find_stored_video_task(lookup).await? else {
+        return Ok(None);
+    };
+
+    if !matches!(task.provider_api_format.as_deref(), Some("openai:video")) {
+        return Ok(None);
+    }
+
+    Ok(Some(map_openai_stored_task_to_read_response(task)))
+}
+
+async fn read_gemini_video_task_response(
+    state: &impl StoredVideoTaskReadSide,
+    request_path: &str,
+) -> Result<Option<LocalVideoTaskReadResponse>, DataLayerError> {
+    let Some(lookup) = resolve_video_task_read_lookup_key(Some("gemini"), request_path) else {
+        return Ok(None);
+    };
+
+    let Some(task) = state.find_stored_video_task(lookup).await? else {
+        return Ok(None);
+    };
+
+    if !matches!(task.provider_api_format.as_deref(), Some("gemini:video")) {
+        return Ok(None);
+    }
+
+    Ok(Some(map_gemini_stored_task_to_read_response(task)))
+}