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/handlers/admin/shared/mod.rs

@@ -0,0 +1,15 @@
+mod paths;
+mod payloads;
+
+pub(crate) use self::paths::*;
+pub(crate) use self::payloads::*;
+pub(crate) use crate::handlers::shared::{
+    attach_admin_audit_response, build_admin_provider_key_response,
+    decrypt_catalog_secret_with_fallbacks, default_provider_key_status_snapshot,
+    effective_catalog_encryption_key, encrypt_catalog_secret_with_fallbacks, json_string_list,
+    masked_catalog_api_key, normalize_json_array, normalize_json_object, normalize_string_list,
+    parse_catalog_auth_config_json, provider_catalog_key_supports_format,
+    provider_key_health_summary, provider_key_status_snapshot_payload, query_param_bool,
+    query_param_optional_bool, query_param_value, unix_secs_to_rfc3339,
+    OFFICIAL_EXTERNAL_MODEL_PROVIDERS,
+};

apps/aether-gateway/src/handlers/admin/shared/paths.rs

@@ -0,0 +1,37 @@
+pub(crate) fn is_admin_gemini_files_mappings_root(request_path: &str) -> bool {
+    matches!(
+        request_path,
+        "/api/admin/gemini-files/mappings" | "/api/admin/gemini-files/mappings/"
+    )
+}
+
+pub(crate) fn is_admin_gemini_files_stats_root(request_path: &str) -> bool {
+    matches!(
+        request_path,
+        "/api/admin/gemini-files/stats" | "/api/admin/gemini-files/stats/"
+    )
+}
+
+pub(crate) fn is_admin_gemini_files_capable_keys_root(request_path: &str) -> bool {
+    matches!(
+        request_path,
+        "/api/admin/gemini-files/capable-keys" | "/api/admin/gemini-files/capable-keys/"
+    )
+}
+
+pub(crate) fn is_admin_gemini_files_upload_root(request_path: &str) -> bool {
+    matches!(
+        request_path,
+        "/api/admin/gemini-files/upload" | "/api/admin/gemini-files/upload/"
+    )
+}
+
+pub(crate) fn admin_gemini_file_mapping_id_from_path(request_path: &str) -> Option<String> {
+    let raw = request_path.strip_prefix("/api/admin/gemini-files/mappings/")?;
+    let normalized = raw.trim().trim_matches('/');
+    if normalized.is_empty() || normalized.contains('/') {
+        None
+    } else {
+        Some(normalized.to_string())
+    }
+}

apps/aether-gateway/src/handlers/admin/shared/payloads.rs

@@ -0,0 +1 @@
+