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-data/src/repository/system.rs

@@ -0,0 +1,22 @@
+#[derive(Debug, Clone, PartialEq, serde::Serialize, serde::Deserialize)]
+pub struct StoredSystemConfigEntry {
+    pub key: String,
+    pub value: serde_json::Value,
+    pub description: Option<String>,
+    pub updated_at_unix_secs: Option<u64>,
+}
+
+#[derive(Debug, Clone, PartialEq, serde::Serialize, serde::Deserialize)]
+pub struct AdminSecurityBlacklistEntry {
+    pub ip_address: String,
+    pub reason: String,
+    pub ttl_seconds: Option<i64>,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, serde::Serialize, serde::Deserialize, Default)]
+pub struct AdminSystemStats {
+    pub total_users: u64,
+    pub active_users: u64,
+    pub total_api_keys: u64,
+    pub total_requests: u64,
+}