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-model-fetch/src/config.rs

@@ -0,0 +1,77 @@
+const MODEL_FETCH_INTERVAL_MINUTES_DEFAULT: u64 = 1440;
+const MODEL_FETCH_INTERVAL_MINUTES_MIN: u64 = 60;
+const MODEL_FETCH_INTERVAL_MINUTES_MAX: u64 = 10080;
+const MODEL_FETCH_STARTUP_DELAY_SECONDS_DEFAULT: u64 = 10;
+
+pub fn model_fetch_interval_minutes() -> u64 {
+    std::env::var("MODEL_FETCH_INTERVAL_MINUTES")
+        .ok()
+        .and_then(|value| value.parse::<u64>().ok())
+        .map(|value| {
+            value.clamp(
+                MODEL_FETCH_INTERVAL_MINUTES_MIN,
+                MODEL_FETCH_INTERVAL_MINUTES_MAX,
+            )
+        })
+        .unwrap_or(MODEL_FETCH_INTERVAL_MINUTES_DEFAULT)
+}
+
+pub fn model_fetch_startup_enabled() -> bool {
+    std::env::var("MODEL_FETCH_STARTUP_ENABLED")
+        .ok()
+        .map(|value| value.trim().eq_ignore_ascii_case("true"))
+        .unwrap_or(true)
+}
+
+pub fn model_fetch_startup_delay_seconds() -> u64 {
+    std::env::var("MODEL_FETCH_STARTUP_DELAY_SECONDS")
+        .ok()
+        .and_then(|value| value.parse::<u64>().ok())
+        .unwrap_or(MODEL_FETCH_STARTUP_DELAY_SECONDS_DEFAULT)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{
+        model_fetch_interval_minutes, model_fetch_startup_delay_seconds,
+        model_fetch_startup_enabled,
+    };
+
+    struct TestEnvVarGuard {
+        key: &'static str,
+        previous: Option<String>,
+    }
+
+    impl Drop for TestEnvVarGuard {
+        fn drop(&mut self) {
+            if let Some(previous) = self.previous.as_deref() {
+                std::env::set_var(self.key, previous);
+            } else {
+                std::env::remove_var(self.key);
+            }
+        }
+    }
+
+    fn set_test_env_var(key: &'static str, value: &str) -> TestEnvVarGuard {
+        let previous = std::env::var(key).ok();
+        std::env::set_var(key, value);
+        TestEnvVarGuard { key, previous }
+    }
+
+    #[test]
+    fn interval_minutes_clamps_to_supported_bounds() {
+        let _interval = set_test_env_var("MODEL_FETCH_INTERVAL_MINUTES", "5");
+        assert_eq!(model_fetch_interval_minutes(), 60);
+
+        let _interval = set_test_env_var("MODEL_FETCH_INTERVAL_MINUTES", "20000");
+        assert_eq!(model_fetch_interval_minutes(), 10080);
+    }
+
+    #[test]
+    fn startup_flags_read_from_environment() {
+        let _enabled = set_test_env_var("MODEL_FETCH_STARTUP_ENABLED", "false");
+        let _delay = set_test_env_var("MODEL_FETCH_STARTUP_DELAY_SECONDS", "3");
+        assert!(!model_fetch_startup_enabled());
+        assert_eq!(model_fetch_startup_delay_seconds(), 3);
+    }
+}