🎉 feat: 建立阶段 0 工程骨架与 OpenAPI 契约机器化（WP-01 起步）

- Makefile：统一命令入口（backend/engine/docker/openapi/generate，含 make help）
- backend：FastAPI + uv 最小工程，统一错误体（契约第 7 节）、ADR-015 平台 ID 生成与校验（uuid-utils）；pytest 7 项、ruff、mypy strict、启动冒烟全部通过
- engine：灵台引擎 C++23/CMake 骨架，按 Lingtai C ABI 契约实现 lingtai.h（13 个 HjStatus、版本协商、句柄表防二次释放、线程局部错误上下文）；doctest 5 用例通过，nm 验证仅导出 hj_* 符号
- openapi：OpenAPI 3.1 单一真源初始版（错误体/分页/Money/首批平台 ID/ContentBlock 全 21 类型/流事件 envelope），Redocly lint 0 错误、bundle 通过
- frontend：Next.js 最小骨架，openapi-typescript 生成 API 类型并通过 strict tsc
- docker：hunjing-postgres（pgvector:17）compose；.env.example 与 config 示例
- ADR-025：登记测试与契约工具链（doctest/libFuzzer/Redocly/openapi-typescript/uuid-utils）
- desktop：Tauri v2 占位与初始化说明（WP-10 启动）

backend/app/core/errors.py

@@ -0,0 +1,76 @@
+"""统一错误体与领域异常（契约见 docs/contracts/api-conventions.md 第 7 节）。
+
+`code` 是稳定机器值；`message` 可本地化；`details` 不稳定但必须脱敏。
+"""
+
+from typing import Any
+
+from fastapi import FastAPI, Request
+from fastapi.exceptions import RequestValidationError
+from fastapi.responses import JSONResponse
+from pydantic import BaseModel
+
+
+class ErrorBody(BaseModel):
+    code: str
+    message: str
+    trace_id: str | None = None
+    request_id: str | None = None
+    details: dict[str, Any] | None = None
+
+
+class ErrorEnvelope(BaseModel):
+    error: ErrorBody
+
+
+class HunjingError(Exception):
+    """领域异常基类：映射为稳定错误码的 HTTP 响应。"""
+
+    status_code: int = 500
+    code: str = "internal"
+
+    def __init__(self, message: str, *, details: dict[str, Any] | None = None) -> None:
+        super().__init__(message)
+        self.message = message
+        self.details = details
+
+
+class ValidationFailed(HunjingError):
+    status_code = 422
+    code = "validation_failed"
+
+
+def _envelope(
+    code: str, message: str, request: Request, details: dict[str, Any] | None
+) -> dict[str, Any]:
+    return {
+        "error": {
+            "code": code,
+            "message": message,
+            "trace_id": None,
+            "request_id": request.headers.get("Hunjing-Request-ID"),
+            "details": details,
+        }
+    }
+
+
+def install_error_handlers(app: FastAPI) -> None:
+    @app.exception_handler(HunjingError)
+    async def _hunjing_error(request: Request, exc: HunjingError) -> JSONResponse:
+        return JSONResponse(
+            status_code=exc.status_code,
+            content=_envelope(exc.code, exc.message, request, exc.details),
+        )
+
+    @app.exception_handler(RequestValidationError)
+    async def _validation_error(request: Request, exc: RequestValidationError) -> JSONResponse:
+        field_errors = [
+            {"path": ".".join(str(p) for p in err["loc"]), "reason": err["type"]}
+            for err in exc.errors()
+        ]
+        return JSONResponse(
+            status_code=422,
+            content=_envelope(
+                "validation_failed", "请求字段不合法。", request, {"field_errors": field_errors}
+            ),
+        )