refactor: 移除 Python upstream 依赖，清理全部 legacy/Python 兼容层

- 移除 upstream_base_url 参数及 AETHER_GATEWAY_UPSTREAM 环境变量，gateway 不再需要指向 Python 宿主
- 删除所有 LEGACY_*/PYTHON_* 常量、路由组、header 定义及 sunset/phaseout 机制
- 将 legacy_gateway_bridge 重命名为 internal_gateway，executor 相关命名统一为 execution_runtime
- dev.sh 新增 Postgres/Redis 预检查，移除 upstream 相关启动参数和提示
- 新增 ai_public 路由处理器
- 全量适配 handler、test、state、control 等模块的命名和接口变更

apps/aether-gateway/src/video_tasks/seed/snapshot.rs

@@ -1,4 +1,13 @@
-use super::*;
+use aether_data::repository::video_tasks::{StoredVideoTask, UpsertVideoTask};
+use serde_json::{json, Map, Value};
+
+use crate::gateway::provider_transport::GatewayProviderTransportSnapshot;
+
+use super::super::{
+    local_status_from_stored, non_empty_owned, request_body_string, GeminiVideoTaskSeed,
+    LocalVideoTaskPersistence, LocalVideoTaskReadResponse, LocalVideoTaskSnapshot,
+    LocalVideoTaskStatus, LocalVideoTaskTransport, OpenAiVideoTaskSeed,
+};
 
 impl LocalVideoTaskSnapshot {
     pub(crate) fn to_upsert_record(&self) -> UpsertVideoTask {