refactor: 移除 Python 后端源码，全面迁移至 Rust gateway 架构

- 删除全部 Python 源码 (src/) 及 Alembic 迁移脚本，归档至 _deprecated_py_src/
- 重构 Rust gateway ai_pipeline: 拆分 planner/finalize 模块，新增 contracts/adaptation 层
- 重组 handlers 模块为 admin/public/proxy/internal/shared 子模块结构
- 新增 executor 模块，引入 Rust 原生数据库迁移 (aether-data/migrations)
- 简化 CI/Docker 构建流程，移除 base image 二级构建，统一为单一 app image
- 移除 Python 相关基础设施文件 (entrypoint.sh, gunicorn_conf.py, Dockerfile.base)

_deprecated_py_src/services/candidate/submit.py

@@ -0,0 +1,66 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Any, Protocol, runtime_checkable
+
+import httpx
+
+from src.services.billing.rule_service import BillingRuleLookupResult
+from src.services.scheduling.aware_scheduler import ProviderCandidate
+
+
+@runtime_checkable
+class SubmitFunc(Protocol):
+    async def __call__(self, candidate: ProviderCandidate) -> httpx.Response: ...
+
+
+@runtime_checkable
+class ExtractExternalTaskIdFunc(Protocol):
+    def __call__(self, payload: dict[str, Any]) -> str | None: ...
+
+
+class UpstreamClientRequestError(RuntimeError):
+    """命中上游终止规则（不应继续 failover）的错误。"""
+
+    def __init__(
+        self,
+        *,
+        response: httpx.Response,
+        candidate_keys: list[dict[str, Any]],
+    ) -> None:
+        self.response = response
+        self.candidate_keys = candidate_keys
+        super().__init__(f"Upstream stop rule matched: HTTP {response.status_code}")
+
+
+class AllCandidatesFailedError(RuntimeError):
+    def __init__(
+        self,
+        *,
+        reason: str,
+        candidate_keys: list[dict[str, Any]],
+        last_status_code: int | None = None,
+    ) -> None:
+        self.reason = reason
+        self.candidate_keys = candidate_keys
+        self.last_status_code = last_status_code
+        super().__init__(f"All candidates failed: {reason}")
+
+
+class CandidateUnsupportedError(RuntimeError):
+    """候选不被当前任务支持（如 auth_type/格式转换需求不支持）。"""
+
+
+class CandidateSubmissionError(RuntimeError):
+    """候选提交异常（网络/解密/解析等）。"""
+
+
+@dataclass(slots=True)
+class SubmitOutcome:
+    candidate: ProviderCandidate
+    candidate_keys: list[dict[str, Any]]
+    external_task_id: str
+    rule_lookup: BillingRuleLookupResult | None
+    upstream_payload: dict[str, Any] | None = None
+    upstream_headers: dict[str, str] | None = None
+    upstream_status_code: int | None = None