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/__init__.py

@@ -0,0 +1,28 @@
+"""
+Candidate domain (Phase2)
+
+This package centralizes:
+- candidate resolving (Provider/Endpoint/Key combinations)
+- request_candidates recording & audit
+- failover execution policies
+"""
+
+from src.services.candidate.policy import RetryMode, RetryPolicy, SkipPolicy
+from src.services.candidate.schema import (
+    CANDIDATE_KEY_SCHEMA_VERSION,
+    CandidateKey,
+    CandidateResult,
+)
+from src.services.candidate.service import CandidateService
+
+__all__ = [
+    "CandidateService",
+    # schema
+    "CANDIDATE_KEY_SCHEMA_VERSION",
+    "CandidateKey",
+    "CandidateResult",
+    # policies
+    "RetryMode",
+    "RetryPolicy",
+    "SkipPolicy",
+]