refactor: 大规模模块拆分与代码精简，新增 ai-pipeline/data-contracts 独立 crate

- 新增 aether-ai-pipeline 和 aether-data-contracts crate，将 pipeline 逻辑与数据契约从 gateway 中解耦
- 重构 admin handlers：拆分单体模块为 auth/billing/endpoint/features/model/observability/provider/system 等独立子模块
- 合并 chat/cli 重复代码路径：精简 conversion、finalize、planner 中的 sync/chat/cli 分支
- 重构 scheduler/executor/data 层，引入 facade 模式降低模块间耦合
- 移除冗余的 intent 模块，将 plan_fallback/policy/stream_path/sync_path 迁移至 executor
- 前端适配：调整 admin API 调用和 provider 模型测试对话框

apps/aether-gateway/src/scheduler/mod.rs

@@ -1,38 +1,3 @@
-mod candidate;
-mod failover;
-mod health;
-mod request_candidate_state;
-mod request_candidates;
-mod route;
-
-pub(crate) use candidate::{
-    list_selectable_candidates,
-    list_selectable_candidates_for_required_capability_without_requested_model,
-    read_cached_scheduler_affinity_target, read_minimal_candidate_selection,
-    GatewayMinimalCandidateSelectionCandidate,
-};
-pub(crate) use candidate::{
-    SchedulerCandidateSelectionRowSource, SchedulerRuntimeState,
-};
-pub(crate) use failover::{
-    resolve_core_stream_direct_finalize_report_kind,
-    resolve_core_stream_error_finalize_report_kind, resolve_core_sync_error_finalize_report_kind,
-    should_fallback_to_control_stream, should_fallback_to_control_sync,
-    should_finalize_sync_response, should_retry_next_local_candidate_stream,
-    should_retry_next_local_candidate_sync,
-};
-pub(crate) use health::{
-    count_recent_rpm_requests_for_provider_key, count_recent_rpm_requests_for_provider_key_since,
-    is_provider_key_circuit_open, provider_key_health_score, provider_key_rpm_allows_request_since,
-    PROVIDER_KEY_RPM_WINDOW_SECS,
-};
-pub(crate) use request_candidate_state::SchedulerRequestCandidateRuntimeState;
-pub(crate) use request_candidates::{
-    current_unix_secs, ensure_execution_request_candidate_slot, execution_error_details,
-    record_local_request_candidate_status, record_report_request_candidate_status,
-};
-pub(crate) use route::{
-    is_matching_stream_request, resolve_execution_runtime_stream_plan_kind,
-    resolve_execution_runtime_sync_plan_kind, supports_stream_scheduler_decision_kind,
-    supports_sync_scheduler_decision_kind,
-};
+pub(crate) mod affinity;
+pub(crate) mod candidate;
+pub(crate) mod state;