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-runtime/src/redaction.rs

@@ -0,0 +1,36 @@
+use std::fmt::Write as _;
+
+use sha2::{Digest, Sha256};
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct TextPayloadSummary {
+    pub bytes: usize,
+    pub sha256: String,
+}
+
+pub fn summarize_text_payload(text: &str) -> TextPayloadSummary {
+    let digest = Sha256::digest(text.as_bytes());
+    let mut sha256 = String::with_capacity(digest.len() * 2);
+    for byte in digest {
+        write!(&mut sha256, "{byte:02x}").expect("writing to string should not fail");
+    }
+    TextPayloadSummary {
+        bytes: text.len(),
+        sha256,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::summarize_text_payload;
+
+    #[test]
+    fn summarizes_text_payload_without_exposing_content() {
+        let summary = summarize_text_payload("secret-body");
+        assert_eq!(summary.bytes, 11);
+        assert_eq!(
+            summary.sha256,
+            "7c3029502007b2beae470d090221f0a8f7708be361be4662f4b649426e5767b3"
+        );
+    }
+}