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/tests/fixtures.rs

@@ -1,4 +1,11 @@
-use super::*;
+use std::collections::BTreeMap;
+
+use serde_json::json;
+
+use super::{
+    ExecutionPlan, ExecutionTimeouts, GatewayControlAuthContext, LocalVideoTaskPersistence,
+    LocalVideoTaskTransport, ProxySnapshot, RequestBody,
+};
 
 pub(super) fn sample_transport(
     base_url: &str,

apps/aether-gateway/src/video_tasks/tests/plans.rs

@@ -1,5 +1,11 @@
 use super::fixtures::*;
-use super::*;
+use serde_json::{json, Value};
+use uuid::Uuid;
+
+use super::{
+    GeminiVideoTaskSeed, LocalVideoTaskSnapshot, LocalVideoTaskStatus, OpenAiVideoTaskSeed,
+    VideoTaskService, VideoTaskTruthSourceMode,
+};
 
 #[test]
 fn rust_authoritative_service_builds_openai_cancel_follow_up_plan() {

apps/aether-gateway/src/video_tasks/tests/projection.rs

@@ -1,5 +1,10 @@
 use super::fixtures::*;
-use super::*;
+use serde_json::{json, Value};
+
+use super::{
+    GeminiVideoTaskSeed, LocalVideoTaskContentAction, LocalVideoTaskSnapshot, LocalVideoTaskStatus,
+    OpenAiVideoTaskSeed, VideoTaskService, VideoTaskTruthSourceMode,
+};
 
 #[test]
 fn rust_authoritative_service_projects_openai_status_into_local_read_response() {

apps/aether-gateway/src/video_tasks/tests/sync.rs

@@ -1,5 +1,10 @@
 use super::fixtures::*;
-use super::*;
+use serde_json::{json, Map, Value};
+
+use super::{
+    GeminiVideoTaskSeed, LocalVideoTaskSeed, LocalVideoTaskSnapshot, LocalVideoTaskStatus,
+    OpenAiVideoTaskSeed, VideoTaskService, VideoTaskSyncReportMode, VideoTaskTruthSourceMode,
+};
 
 #[test]
 fn openai_video_seed_preserves_existing_local_identity() {