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-provider-transport/src/antigravity/request.rs

@@ -0,0 +1,120 @@
+use serde_json::{Map, Value};
+
+use super::auth::{AntigravityRequestAuth, ANTIGRAVITY_REQUEST_USER_AGENT};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum AntigravityEnvelopeRequestType {
+    Agent,
+    EndpointTest,
+}
+
+impl AntigravityEnvelopeRequestType {
+    fn as_str(self) -> &'static str {
+        match self {
+            Self::Agent => "agent",
+            Self::EndpointTest => "endpoint_test",
+        }
+    }
+}
+
+#[derive(Debug, Clone, PartialEq)]
+pub enum AntigravityRequestEnvelopeSupport {
+    Supported(Value),
+    Unsupported(AntigravityRequestEnvelopeUnsupportedReason),
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum AntigravityRequestEnvelopeUnsupportedReason {
+    NonObjectBody,
+    MissingContents,
+    MissingRequestId,
+    MissingModel,
+    ComplexEnvelopeTransform,
+}
+
+pub fn classify_antigravity_safe_request_body(
+    request_body: &Value,
+) -> Result<(), AntigravityRequestEnvelopeUnsupportedReason> {
+    let Value::Object(map) = request_body else {
+        return Err(AntigravityRequestEnvelopeUnsupportedReason::NonObjectBody);
+    };
+    if !map.contains_key("contents") {
+        return Err(AntigravityRequestEnvelopeUnsupportedReason::MissingContents);
+    }
+    if contains_blocked_request_features(request_body) {
+        return Err(AntigravityRequestEnvelopeUnsupportedReason::ComplexEnvelopeTransform);
+    }
+
+    Ok(())
+}
+
+pub fn build_antigravity_safe_v1internal_request(
+    auth: &AntigravityRequestAuth,
+    request_id: &str,
+    model: &str,
+    request_body: &Value,
+    request_type: AntigravityEnvelopeRequestType,
+) -> AntigravityRequestEnvelopeSupport {
+    if request_id.trim().is_empty() {
+        return AntigravityRequestEnvelopeSupport::Unsupported(
+            AntigravityRequestEnvelopeUnsupportedReason::MissingRequestId,
+        );
+    }
+    if model.trim().is_empty() {
+        return AntigravityRequestEnvelopeSupport::Unsupported(
+            AntigravityRequestEnvelopeUnsupportedReason::MissingModel,
+        );
+    }
+    if let Err(reason) = classify_antigravity_safe_request_body(request_body) {
+        return AntigravityRequestEnvelopeSupport::Unsupported(reason);
+    }
+
+    let Value::Object(source) = request_body else {
+        return AntigravityRequestEnvelopeSupport::Unsupported(
+            AntigravityRequestEnvelopeUnsupportedReason::NonObjectBody,
+        );
+    };
+
+    let mut inner_request: Map<String, Value> = source.clone();
+    inner_request.remove("model");
+    inner_request.remove("safetySettings");
+    inner_request.remove("safety_settings");
+
+    AntigravityRequestEnvelopeSupport::Supported(serde_json::json!({
+        "project": auth.project_id,
+        "requestId": request_id,
+        "request": Value::Object(inner_request),
+        "model": model,
+        "userAgent": ANTIGRAVITY_REQUEST_USER_AGENT,
+        "requestType": request_type.as_str(),
+    }))
+}
+
+fn contains_blocked_request_features(value: &Value) -> bool {
+    match value {
+        Value::Object(map) => map.iter().any(|(key, inner)| {
+            is_blocked_request_key(key.as_str()) || contains_blocked_request_features(inner)
+        }),
+        Value::Array(items) => items.iter().any(contains_blocked_request_features),
+        _ => false,
+    }
+}
+
+fn is_blocked_request_key(key: &str) -> bool {
+    matches!(
+        key.trim(),
+        "systemInstruction"
+            | "system_instruction"
+            | "tools"
+            | "toolConfig"
+            | "tool_config"
+            | "thinkingConfig"
+            | "thinking_config"
+            | "imageConfig"
+            | "image_config"
+            | "functionCall"
+            | "function_call"
+            | "functionResponse"
+            | "function_response"
+    )
+}