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-billing/src/lib.rs

@@ -1,4 +1,5 @@
 mod default_rule;
+mod event_enrichment;
 mod formula_engine;
 mod models;
 mod precision;
@@ -6,14 +7,17 @@ mod pricing;
 mod schema;
 mod service;
 mod token_normalization;
-mod usage_mapper;
 
+pub use aether_usage_runtime::{
+    map_usage, map_usage_from_response, StandardizedUsage, UsageMapper,
+};
 pub use default_rule::{normalize_task_type, DefaultBillingRuleGenerator, VirtualBillingRule};
+pub use event_enrichment::{enrich_usage_event_with_billing, BillingModelContextLookup};
 pub use formula_engine::{
     extract_variable_names, BillingIncompleteError, ExpressionEvaluationError, FormulaEngine,
     FormulaEvaluationResult, FormulaEvaluationStatus, UnsafeExpressionError,
 };
-pub use models::{BillingDimension, BillingUnit, CostBreakdown, StandardizedUsage};
+pub use models::{BillingDimension, BillingUnit, CostBreakdown};
 pub use precision::{
     quantize_cost, quantize_display, quantize_value, BILLING_DISPLAY_PRECISION,
     BILLING_STORAGE_PRECISION,
@@ -24,4 +28,3 @@ pub use schema::{
 };
 pub use service::BillingService;
 pub use token_normalization::normalize_input_tokens_for_billing;
-pub use usage_mapper::{map_usage, map_usage_from_response, UsageMapper};