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/gemini.rs

@@ -1,4 +1,15 @@
-use super::*;
+use aether_contracts::{ExecutionPlan, RequestBody};
+use aether_data::repository::video_tasks::UpsertVideoTask;
+use serde_json::{json, Map, Value};
+
+use crate::gateway::GatewayControlAuthContext;
+
+use super::super::{
+    build_video_follow_up_report_context, current_unix_timestamp_secs, gemini_metadata_video_url,
+    request_body_string, request_body_u32, resolve_follow_up_auth, GeminiVideoTaskSeed,
+    LocalVideoTaskFollowUpPlan, LocalVideoTaskSnapshot, LocalVideoTaskStatus,
+    DEFAULT_VIDEO_TASK_MAX_POLL_COUNT, DEFAULT_VIDEO_TASK_POLL_INTERVAL_SECONDS,
+};
 
 impl GeminiVideoTaskSeed {
     pub(crate) fn apply_provider_body(&mut self, provider_body: &Map<String, Value>) {