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-usage-runtime/src/lib.rs

@@ -0,0 +1,44 @@
+pub mod config;
+pub mod event;
+pub mod queue;
+pub mod record;
+pub mod report;
+pub mod report_context;
+pub mod runtime;
+pub mod settlement;
+pub mod standardized_usage;
+pub mod usage_mapper;
+pub mod worker;
+pub mod write;
+
+pub use config::UsageRuntimeConfig;
+pub use event::{now_ms, UsageEvent, UsageEventData, UsageEventType, USAGE_EVENT_VERSION};
+pub use queue::UsageQueue;
+pub use record::build_upsert_usage_record_from_event;
+pub use report::{
+    extract_gemini_file_mapping_entries, gemini_file_mapping_cache_key,
+    infer_internal_finalize_signature, is_local_ai_stream_report_kind,
+    is_local_ai_sync_report_kind, normalize_gemini_file_name, report_request_id,
+    resolve_internal_finalize_route, should_handle_local_stream_report,
+    should_handle_local_sync_report, sync_report_represents_failure, GatewayStreamReportRequest,
+    GatewaySyncReportRequest, GeminiFileMappingEntry, InternalFinalizeRoute,
+    GEMINI_FILE_MAPPING_TTL_SECONDS,
+};
+pub use report_context::{
+    build_locally_actionable_report_context_from_request_candidate,
+    build_locally_actionable_report_context_from_video_task, report_context_is_locally_actionable,
+};
+pub use runtime::{UsageBillingEventEnricher, UsageRuntime, UsageRuntimeAccess};
+pub use settlement::{settle_usage_if_needed, UsageSettlementWriter};
+pub use standardized_usage::StandardizedUsage;
+pub use usage_mapper::{map_usage, map_usage_from_response, UsageMapper};
+pub use worker::{
+    build_usage_queue_worker, write_event_record, UsageDataEventRecorder, UsageEventRecorder,
+    UsageQueueWorker, UsageRecordWriter,
+};
+pub use write::{
+    build_pending_usage_record, build_stream_terminal_usage_event,
+    build_stream_terminal_usage_outcome, build_streaming_usage_record,
+    build_sync_terminal_usage_event, build_sync_terminal_usage_outcome,
+    build_terminal_usage_event_from_outcome, TerminalUsageOutcome, UsageTerminalState,
+};