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/submit/prepare.py

@@ -0,0 +1,126 @@
+from __future__ import annotations
+
+from collections.abc import Callable
+from dataclasses import dataclass
+from typing import Any
+
+from sqlalchemy.orm import Session
+
+from src.core.logger import logger
+from src.models.database import ApiKey
+from src.services.candidate.resolver import CandidateResolver
+from src.services.candidate.submit import AllCandidatesFailedError
+from src.services.scheduling.aware_scheduler import get_cache_aware_scheduler
+from src.services.system.config import SystemConfigService
+from src.services.task.submit import ApplyPoolReorderFn, ExpandPoolCandidatesFn
+
+
+@dataclass(slots=True)
+class PreparedSubmitCandidates:
+    """异步提交前的候选准备结果。"""
+
+    candidates: list[Any]
+    record_map: dict[tuple[int, int], str]
+
+
+class AsyncSubmitPreparationService:
+    """异步提交候选准备服务。"""
+
+    def __init__(
+        self,
+        db: Session,
+        redis_client: Any | None,
+        *,
+        sanitize: Callable[[str], str],
+    ) -> None:
+        self.db = db
+        self.redis = redis_client
+        self._sanitize = sanitize
+
+    async def prepare_candidates(
+        self,
+        *,
+        api_format: str,
+        model_name: str,
+        affinity_key: str,
+        user_api_key: ApiKey,
+        request_id: str | None,
+        capability_requirements: dict[str, bool] | None,
+        request_body: dict[str, Any] | None,
+        max_candidates: int | None,
+        apply_pool_reorder: ApplyPoolReorderFn,
+        expand_pool_candidates_for_async_submit: ExpandPoolCandidatesFn,
+    ) -> PreparedSubmitCandidates:
+        priority_mode = SystemConfigService.get_config(
+            self.db,
+            "provider_priority_mode",
+            "provider",
+        )
+        scheduling_mode = SystemConfigService.get_config(
+            self.db,
+            "scheduling_mode",
+            "cache_affinity",
+        )
+        cache_scheduler = await get_cache_aware_scheduler(
+            self.redis,
+            priority_mode=priority_mode,
+            scheduling_mode=scheduling_mode,
+        )
+        resolver = CandidateResolver(db=self.db, cache_scheduler=cache_scheduler)
+
+        candidates, _global_model_id = await resolver.fetch_candidates(
+            api_format=api_format,
+            model_name=model_name,
+            affinity_key=affinity_key,
+            user_api_key=user_api_key,
+            request_id=request_id,
+            is_stream=False,
+            capability_requirements=capability_requirements,
+            request_body=request_body,
+        )
+        _ = _global_model_id
+
+        if not candidates:
+            raise AllCandidatesFailedError(
+                reason="no_candidates",
+                candidate_keys=[],
+                last_status_code=None,
+            )
+
+        # 号池排序涉及大量 Redis 操作，提前释放 DB 连接避免连接池压力
+        from src.services.scheduling.utils import release_db_connection_before_await
+
+        release_db_connection_before_await(self.db)
+
+        # Account Pool: keep internal key failover order/skip behavior
+        # consistent with the SYNC path.
+        candidates, _pool_traces = await apply_pool_reorder(
+            candidates,
+            request_body=request_body,
+        )
+        _ = _pool_traces
+        candidates = expand_pool_candidates_for_async_submit(candidates)
+
+        if max_candidates is not None and max_candidates > 0:
+            candidates = candidates[:max_candidates]
+
+        # Pre-create RequestCandidate records (no retry expand for async submit stage)
+        record_map: dict[tuple[int, int], str] = {}
+        if request_id:
+            try:
+                record_map = await resolver.create_candidate_records_async(
+                    all_candidates=candidates,
+                    request_id=request_id,
+                    user_id=str(user_api_key.user_id),
+                    user_api_key=user_api_key,
+                    required_capabilities=capability_requirements,
+                    expand_retries=False,
+                )
+            except Exception as exc:
+                logger.warning(
+                    "[TaskService] Failed to create candidate records: {}",
+                    self._sanitize(str(exc)),
+                )
+                record_map = {}
+
+        return PreparedSubmitCandidates(candidates=candidates, record_map=record_map)