feat(gateway/data): 新增 usage 关键词搜索、缓存命中摘要、结算成本摘要及 dashboard 聚合查询能力

- 新增 UsageAuditKeywordSearchQuery，支持多关键词、用户名、API Key 交叉过滤
- 新增 UsageCacheHitSummaryQuery/StoredUsageCacheHitSummary，统计请求缓存命中率
- 新增 UsageSettledCostSummaryQuery/StoredUsageSettledCostSummary，汇总结算成本
- 新增 UsageDashboardSummaryQuery、UsageBreakdownSummaryQuery 等 dashboard 聚合类型
- SQL 层实现对应查询方法，含 list_by_ids、list_usage_audits_by_keyword_search、count_usage_audits_by_keyword_search
- 将上述能力通过 GatewayDataState / AdminAppState 暴露给 handler 层
- user_me_usage 及 dashboard_filters 切换为新查询接口，移除旧的内存过滤逻辑
- 同步更新 memory 层及测试

apps/aether-gateway/src/handlers/admin/observability/mod.rs

@@ -6,8 +6,8 @@ mod usage;
 pub(super) use self::monitoring::maybe_build_local_admin_monitoring_response;
 pub(super) use self::routes::maybe_build_local_admin_observability_response;
 pub(crate) use self::stats::{
-    admin_stats_bad_request_response, list_usage_for_optional_range, list_usage_for_range,
-    maybe_build_local_admin_stats_response, parse_bounded_u32, round_to,
+    admin_stats_bad_request_response, maybe_build_local_admin_stats_response, parse_bounded_u32,
+    round_to,
 };
 pub(crate) use self::stats::{AdminStatsTimeRange, AdminStatsUsageFilter};
 pub(crate) use self::usage::maybe_build_local_admin_usage_response;