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/policy.rs

@@ -0,0 +1,113 @@
+use serde_json::Value;
+
+use super::super::snapshot::GatewayProviderTransportSnapshot;
+use super::auth::{
+    resolve_local_antigravity_request_auth, AntigravityRequestAuth, AntigravityRequestAuthSupport,
+    AntigravityRequestAuthUnsupportedReason, ANTIGRAVITY_PROVIDER_TYPE,
+};
+use super::request::{
+    classify_antigravity_safe_request_body, AntigravityEnvelopeRequestType,
+    AntigravityRequestEnvelopeUnsupportedReason,
+};
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct AntigravityRequestSideSpec {
+    pub auth: AntigravityRequestAuth,
+    pub request_type: AntigravityEnvelopeRequestType,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum AntigravityRequestSideSupport {
+    Supported(AntigravityRequestSideSpec),
+    Unsupported(AntigravityRequestSideUnsupportedReason),
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum AntigravityRequestSideUnsupportedReason {
+    InactiveTransport,
+    WrongProviderType,
+    UnsupportedApiFormat,
+    UnsupportedCustomPath,
+    UnsupportedHeaderRules,
+    UnsupportedBodyRules,
+    UnsupportedNetworkConfig,
+    UnsupportedAuth(AntigravityRequestAuthUnsupportedReason),
+    UnsupportedEnvelope(AntigravityRequestEnvelopeUnsupportedReason),
+}
+
+pub fn classify_local_antigravity_request_support(
+    transport: &GatewayProviderTransportSnapshot,
+    request_body: &Value,
+    request_type: AntigravityEnvelopeRequestType,
+) -> AntigravityRequestSideSupport {
+    if !transport.provider.is_active || !transport.endpoint.is_active || !transport.key.is_active {
+        return AntigravityRequestSideSupport::Unsupported(
+            AntigravityRequestSideUnsupportedReason::InactiveTransport,
+        );
+    }
+    if !transport
+        .provider
+        .provider_type
+        .trim()
+        .eq_ignore_ascii_case(ANTIGRAVITY_PROVIDER_TYPE)
+    {
+        return AntigravityRequestSideSupport::Unsupported(
+            AntigravityRequestSideUnsupportedReason::WrongProviderType,
+        );
+    }
+
+    let endpoint_format = transport.endpoint.api_format.trim();
+    if !endpoint_format.eq_ignore_ascii_case("gemini:chat")
+        && !endpoint_format.eq_ignore_ascii_case("gemini:cli")
+    {
+        return AntigravityRequestSideSupport::Unsupported(
+            AntigravityRequestSideUnsupportedReason::UnsupportedApiFormat,
+        );
+    }
+    if transport
+        .endpoint
+        .custom_path
+        .as_deref()
+        .is_some_and(|value| !value.trim().is_empty())
+    {
+        return AntigravityRequestSideSupport::Unsupported(
+            AntigravityRequestSideUnsupportedReason::UnsupportedCustomPath,
+        );
+    }
+    if transport.endpoint.header_rules.is_some() {
+        return AntigravityRequestSideSupport::Unsupported(
+            AntigravityRequestSideUnsupportedReason::UnsupportedHeaderRules,
+        );
+    }
+    if transport.endpoint.body_rules.is_some() {
+        return AntigravityRequestSideSupport::Unsupported(
+            AntigravityRequestSideUnsupportedReason::UnsupportedBodyRules,
+        );
+    }
+    if transport.provider.proxy.is_some()
+        || transport.endpoint.proxy.is_some()
+        || transport.key.proxy.is_some()
+        || transport.key.fingerprint.is_some()
+    {
+        return AntigravityRequestSideSupport::Unsupported(
+            AntigravityRequestSideUnsupportedReason::UnsupportedNetworkConfig,
+        );
+    }
+
+    let auth = match resolve_local_antigravity_request_auth(transport) {
+        AntigravityRequestAuthSupport::Supported(auth) => auth,
+        AntigravityRequestAuthSupport::Unsupported(reason) => {
+            return AntigravityRequestSideSupport::Unsupported(
+                AntigravityRequestSideUnsupportedReason::UnsupportedAuth(reason),
+            );
+        }
+    };
+
+    if let Err(reason) = classify_antigravity_safe_request_body(request_body) {
+        return AntigravityRequestSideSupport::Unsupported(
+            AntigravityRequestSideUnsupportedReason::UnsupportedEnvelope(reason),
+        );
+    }
+
+    AntigravityRequestSideSupport::Supported(AntigravityRequestSideSpec { auth, request_type })
+}