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/precision.py

@@ -0,0 +1,54 @@
+"""
+Precision helpers for billing calculations.
+
+We standardize money arithmetic with `Decimal` and quantize to stable precisions.
+
+Notes:
+- `Decimal` context precision (`DECIMAL_CONTEXT_PRECISION`) is **significant digits**,
+  not "decimal places".
+- We keep these as constants (not runtime-configurable) to avoid drift between
+  environments during billing reconciliation.
+"""
+
+from __future__ import annotations
+
+from decimal import ROUND_HALF_UP, Decimal, localcontext
+
+# Decimal context precision (significant digits)
+DECIMAL_CONTEXT_PRECISION = 28
+
+# Money precisions
+BILLING_STORAGE_PRECISION = 8  # persisted to DB / metadata
+BILLING_DISPLAY_PRECISION = 6  # UI display
+
+
+def to_decimal(value: float | int | str | Decimal | None) -> Decimal:
+    """Convert values to Decimal safely (float via str to avoid binary artifacts)."""
+    if value is None:
+        return Decimal("0")
+    if isinstance(value, Decimal):
+        return value
+    return Decimal(str(value))
+
+
+def to_money_decimal(value: float | int | str | Decimal | None) -> Decimal:
+    """Convert values to Decimal and quantize to billing storage precision."""
+    return quantize_cost(to_decimal(value))
+
+
+def quantize_decimal(value: Decimal, *, precision: int) -> Decimal:
+    """Quantize a Decimal to the given number of decimal places (ROUND_HALF_UP)."""
+    quantizer = Decimal(10) ** -precision
+    with localcontext() as ctx:
+        ctx.prec = DECIMAL_CONTEXT_PRECISION
+        return value.quantize(quantizer, rounding=ROUND_HALF_UP)
+
+
+def quantize_cost(value: Decimal) -> Decimal:
+    """Quantize to storage precision."""
+    return quantize_decimal(value, precision=BILLING_STORAGE_PRECISION)
+
+
+def quantize_display(value: Decimal) -> Decimal:
+    """Quantize to display precision."""
+    return quantize_decimal(value, precision=BILLING_DISPLAY_PRECISION)