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/schema.py

@@ -0,0 +1,73 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Any
+
+from src.services.scheduling.aware_scheduler import ProviderCandidate
+
+CANDIDATE_KEY_SCHEMA_VERSION = "1.0"
+
+
+@dataclass(frozen=True)
+class CandidateKey:
+    """Stable candidate key snapshot for audit."""
+
+    schema_version: str = CANDIDATE_KEY_SCHEMA_VERSION
+
+    candidate_index: int = 0
+    retry_index: int = 0
+
+    provider_id: str | None = None
+    provider_name: str | None = None
+    endpoint_id: str | None = None
+    key_id: str | None = None
+    key_name: str | None = None
+    auth_type: str | None = None
+    priority: int | None = None
+    is_cached: bool = False
+
+    status: str = "pending"  # pending/success/failed/skipped/available/...
+    skip_reason: str | None = None
+    error_type: str | None = None
+    error_message: str | None = None
+    status_code: int | None = None
+    latency_ms: int | None = None
+    extra_data: dict[str, Any] | None = None
+
+    def to_dict(self) -> dict[str, Any]:
+        data: dict[str, Any] = {
+            "schema_version": self.schema_version,
+            "candidate_index": self.candidate_index,
+            "retry_index": self.retry_index,
+            "provider_id": self.provider_id,
+            "provider_name": self.provider_name,
+            "endpoint_id": self.endpoint_id,
+            "key_id": self.key_id,
+            "key_name": self.key_name,
+            "auth_type": self.auth_type,
+            "priority": self.priority,
+            "is_cached": self.is_cached,
+            "status": self.status,
+            "skip_reason": self.skip_reason,
+            "error_type": self.error_type,
+            "error_message": self.error_message,
+            "status_code": self.status_code,
+            "latency_ms": self.latency_ms,
+            "extra_data": self.extra_data,
+        }
+        # drop Nones for compact audit payload
+        return {k: v for k, v in data.items() if v is not None}
+
+
+@dataclass(slots=True)
+class CandidateResult:
+    """Failover execution result."""
+
+    success: bool
+    selected: ProviderCandidate | None
+    selected_index: int | None
+    candidate_keys: list[CandidateKey]
+
+    external_task_id: str | None = None
+    error: Exception | None = None
+    last_status_code: int | None = None