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/schema.py

@@ -0,0 +1,112 @@
+"""
+Billing schema (stable contracts)
+
+These dataclasses are meant to be stored in `Usage.request_metadata` / `Task.request_metadata`
+for auditability. They are internal-only and MUST NOT be exposed to end users without sanitizing.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from typing import Any, Literal
+
+BILLING_SNAPSHOT_SCHEMA_VERSION = "2.0"
+
+BillingSnapshotStatus = Literal["complete", "incomplete", "no_rule", "legacy"]
+
+
+@dataclass(frozen=True)
+class BillingSnapshot:
+    """
+    Stable billing snapshot for audit.
+
+    v2.0 semantics:
+    - resolved_dimensions: final dimension values used (tokens, request_count, etc.)
+    - resolved_variables: final variables used (prices, tier-resolved values, etc.)
+    - cost_breakdown: itemized costs (quantized)
+    - total_cost: quantized total cost (equals sum(cost_breakdown) when breakdown present)
+
+    Backward compatibility:
+    - dimensions_used aliases resolved_dimensions
+    - cost aliases total_cost
+    """
+
+    schema_version: str = BILLING_SNAPSHOT_SCHEMA_VERSION
+
+    # Rule info (optional for legacy/no_rule)
+    rule_id: str | None = None
+    rule_name: str | None = None
+    scope: str | None = None
+
+    # Rule expression (internal, do not expose to clients)
+    expression: str | None = None
+
+    # v2: resolved inputs
+    resolved_dimensions: dict[str, Any] = field(default_factory=dict)
+    resolved_variables: dict[str, Any] = field(default_factory=dict)
+
+    # v2: breakdown and totals
+    cost_breakdown: dict[str, float] = field(default_factory=dict)
+    total_cost: float = 0.0
+
+    # Tier info (optional)
+    tier_index: int | None = None
+    tier_info: dict[str, Any] | None = None
+
+    # Missing dims
+    missing_required: list[str] = field(default_factory=list)
+
+    # Result status
+    status: BillingSnapshotStatus = "no_rule"
+
+    # Audit
+    calculated_at: str = ""  # ISO 8601
+    engine_version: str = "2.0"
+
+    # ---------------------------------------------------------------------
+    # Backward-compatible aliases (v1 fields)
+    # ---------------------------------------------------------------------
+    @property
+    def dimensions_used(self) -> dict[str, Any]:
+        return self.resolved_dimensions
+
+    @property
+    def cost(self) -> float:
+        return self.total_cost
+
+    def to_dict(self) -> dict[str, Any]:
+        """
+        Serialize snapshot.
+
+        Includes both v2 keys and v1-compatible keys for safer rollouts.
+        """
+        return {
+            "schema_version": self.schema_version,
+            "rule_id": self.rule_id,
+            "rule_name": self.rule_name,
+            "scope": self.scope,
+            "expression": self.expression,
+            # v2
+            "resolved_dimensions": self.resolved_dimensions,
+            "resolved_variables": self.resolved_variables,
+            "cost_breakdown": self.cost_breakdown,
+            "total_cost": self.total_cost,
+            "tier_index": self.tier_index,
+            "tier_info": self.tier_info,
+            "missing_required": self.missing_required,
+            "status": self.status,
+            "calculated_at": self.calculated_at,
+            "engine_version": self.engine_version,
+            # v1 compat
+            "dimensions_used": self.resolved_dimensions,
+            "cost": self.total_cost,
+        }
+
+
+@dataclass(frozen=True)
+class CostResult:
+    """Billing calculation output."""
+
+    cost: float
+    status: BillingSnapshotStatus
+    snapshot: BillingSnapshot