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/lib.rs

@@ -0,0 +1,50 @@
+pub mod antigravity;
+pub mod auth;
+mod auth_config;
+mod cache;
+pub mod claude_code;
+mod generic_oauth;
+mod headers;
+pub mod kiro;
+mod network;
+pub mod oauth_refresh;
+pub mod policy;
+pub mod provider_types;
+pub mod rules;
+pub mod snapshot;
+pub mod url;
+pub mod vertex;
+mod video;
+
+pub use auth::{build_passthrough_headers, ensure_upstream_auth_header};
+pub use cache::{provider_transport_snapshot_looks_refreshed, ProviderTransportSnapshotCacheKey};
+pub use generic_oauth::{
+    supports_local_generic_oauth_request_auth_resolution, GenericOAuthRefreshAdapter,
+};
+pub use headers::{should_skip_request_header, should_skip_upstream_passthrough_header};
+pub use network::{
+    resolve_transport_execution_timeouts, resolve_transport_proxy_snapshot,
+    resolve_transport_proxy_snapshot_with_tunnel_affinity, resolve_transport_tls_profile,
+    transport_proxy_is_locally_supported, TransportTunnelAffinityLookup,
+    TransportTunnelAttachmentOwner,
+};
+pub use oauth_refresh::{
+    supports_local_oauth_request_auth_resolution, CachedOAuthEntry, LocalOAuthRefreshCoordinator,
+    LocalOAuthRefreshError, LocalResolvedOAuthRequestAuth,
+};
+pub use policy::{
+    supports_local_gemini_transport, supports_local_gemini_transport_with_network,
+    supports_local_standard_transport,
+};
+pub use rules::{
+    apply_local_body_rules, apply_local_header_rules, body_rules_are_locally_supported,
+    header_rules_are_locally_supported,
+};
+pub use snapshot::{
+    read_provider_transport_snapshot, GatewayProviderTransportSnapshot,
+    ProviderTransportSnapshotSource,
+};
+pub use video::{
+    reconstruct_local_video_task_snapshot, resolve_local_video_task_transport,
+    VideoTaskTransportSnapshotLookup,
+};