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/task/execute/state_transition.py

@@ -0,0 +1,99 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import TYPE_CHECKING, Any
+
+from src.services.candidate.policy import FailoverAction
+from src.services.task.execute.exception_classification import CandidateErrorAction
+from src.services.task.request_state import RequestBodyState
+
+if TYPE_CHECKING:
+    from src.services.task.execute.failure import TaskFailureOperationsService
+
+
+@dataclass(slots=True)
+class ExecutionErrorTransition:
+    """执行异常后的状态流转决策。"""
+
+    failover_action: FailoverAction
+    max_retries: int | None = None
+
+    def as_failover_tuple(self) -> tuple[FailoverAction, int | None]:
+        return (self.failover_action, self.max_retries)
+
+
+@dataclass(slots=True)
+class SyncExecutionState:
+    """同步执行阶段状态容器（候选上下文 + 异常上下文）。"""
+
+    candidate_record_map: dict[tuple[int, int], str]
+    request_body_state: RequestBodyState | None
+    last_error: Exception | None = None
+    last_candidate: Any | None = None
+
+    def touch_candidate(self, candidate: Any) -> None:
+        self.last_candidate = candidate
+
+    def track_execution_error(self, *, exec_err: Any, candidate: Any) -> None:
+        self.last_candidate = candidate
+        cause = getattr(exec_err, "cause", None)
+        self.last_error = cause if isinstance(cause, Exception) else None
+
+    def resolve_candidate_record_id(self, *, candidate_index: int, record_id: str | None) -> str:
+        if record_id:
+            return str(record_id)
+        return str(self.candidate_record_map.get((candidate_index, 0), "") or "")
+
+    def consume_rectify_retry_extension(
+        self, *, max_retries_for_candidate: int, retry_index: int
+    ) -> int | None:
+        if not self.request_body_state:
+            return None
+        if not self.request_body_state.consume_rectified_this_turn():
+            return None
+        return max(max_retries_for_candidate, retry_index + 2)
+
+    def raise_classified_error(
+        self,
+        *,
+        fallback_error: Any,
+        failure_ops: TaskFailureOperationsService,
+        model_name: str,
+        api_format: str,
+    ) -> None:
+        if self.last_error is not None:
+            failure_ops.attach_metadata_to_error(
+                self.last_error,
+                self.last_candidate,
+                model_name,
+                api_format,
+            )
+            raise self.last_error
+
+        if isinstance(fallback_error, Exception):
+            raise fallback_error
+
+        raise RuntimeError("execution_error_handler requested raise without exception context")
+
+
+def resolve_execution_error_transition(
+    *,
+    action: CandidateErrorAction,
+    state: SyncExecutionState,
+    max_retries_for_candidate: int,
+    retry_index: int,
+) -> ExecutionErrorTransition:
+    """根据异常动作分类，返回 FailoverEngine 可消费的状态流转结果。"""
+    if action == CandidateErrorAction.RETRY_CURRENT:
+        return ExecutionErrorTransition(
+            failover_action=FailoverAction.RETRY,
+            max_retries=state.consume_rectify_retry_extension(
+                max_retries_for_candidate=max_retries_for_candidate,
+                retry_index=retry_index,
+            ),
+        )
+
+    return ExecutionErrorTransition(
+        failover_action=FailoverAction.CONTINUE,
+        max_retries=None,
+    )