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/billing/__init__.py

@@ -0,0 +1,57 @@
+"""
+计费模块
+
+提供配置驱动的计费计算，支持不同厂商的差异化计费模式：
+- Claude: input + output + cache_creation + cache_read
+- OpenAI: input + output + cache_read (无缓存创建费用)
+- 豆包: input + output + cache_read + cache_storage (缓存按时计费)
+- 按次计费: per_request
+
+使用方式:
+    from src.services.billing import BillingCalculator, UsageMapper, StandardizedUsage
+
+    # 1. 将原始 usage 映射为标准格式
+    usage = UsageMapper.map(raw_usage, api_format="openai:chat")
+
+    # 2. 使用计费计算器计算费用
+    calculator = BillingCalculator(template="openai")
+    result = calculator.calculate(usage, prices)
+
+    # 3. 获取费用明细
+    print(result.total_cost)
+    print(result.costs)  # {"input": 0.01, "output": 0.02, ...}
+"""
+
+from src.services.billing.calculator import BillingCalculator, calculate_request_cost
+from src.services.billing.models import (
+    BillingDimension,
+    BillingUnit,
+    CostBreakdown,
+    StandardizedUsage,
+)
+from src.services.billing.schema import BillingSnapshot, CostResult
+from src.services.billing.service import BillingService
+from src.services.billing.templates import BILLING_TEMPLATE_REGISTRY, BillingTemplates
+from src.services.billing.usage_mapper import UsageMapper, map_usage, map_usage_from_response
+
+__all__ = [
+    # 数据模型
+    "BillingDimension",
+    "BillingUnit",
+    "CostBreakdown",
+    "StandardizedUsage",
+    # 模板
+    "BillingTemplates",
+    "BILLING_TEMPLATE_REGISTRY",
+    # 计算器
+    "BillingCalculator",
+    "calculate_request_cost",
+    # 统一入口
+    "BillingService",
+    "BillingSnapshot",
+    "CostResult",
+    # 映射器
+    "UsageMapper",
+    "map_usage",
+    "map_usage_from_response",
+]