fix(HEL-191): 补回问天解势知识文件兜底并消除 Failed to fetch

宿主机 data 挂载会遮盖镜像内 heaven_knowledge.json；增加不受挂载影响的 seed，
并将缺文件/坏 JSON 转为结构化中文错误，前端展示可读提示而非 Failed to fetch。

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: multica-agent <github@multica.ai>

backend/features/heaven/http.py

@@ -3,15 +3,24 @@ from __future__ import annotations
 import json
 from http import HTTPStatus
 
+from backend.features.heaven.knowledge import HeavenKnowledgeError
+
 
 class HeavenHttpMixin:
+    def _send_heaven_client_error(self, exc: Exception) -> None:
+        payload: dict = {"error": str(exc)}
+        code = getattr(exc, "error_code", None)
+        if code:
+            payload["code"] = str(code)
+        self.send_json(payload, HTTPStatus.BAD_REQUEST)
+
     def heaven_hexagram(self) -> None:
         try:
             body = self.read_json_body()
             result = self.application_service.heaven_hexagram(body.get("lines"))
             self.send_json({"ok": True, "hexagram": result})
         except (ValueError, json.JSONDecodeError) as exc:
-            self.send_json({"error": str(exc)}, HTTPStatus.BAD_REQUEST)
+            self._send_heaven_client_error(exc)
 
     def heaven_personal(self) -> None:
         try:
@@ -19,12 +28,12 @@ class HeavenHttpMixin:
             result = self.application_service.heaven_personal(body)
             self.send_json({"ok": True, "personal": result})
         except (ValueError, json.JSONDecodeError) as exc:
-            self.send_json({"error": str(exc)}, HTTPStatus.BAD_REQUEST)
+            self._send_heaven_client_error(exc)
 
     def heaven_interpret(self) -> None:
         try:
             body = self.read_json_body()
             result = self.application_service.heaven_interpret(body)
             self.send_json({"ok": True, **result})
-        except (ValueError, json.JSONDecodeError) as exc:
-            self.send_json({"error": str(exc)}, HTTPStatus.BAD_REQUEST)
+        except (HeavenKnowledgeError, ValueError, json.JSONDecodeError) as exc:
+            self._send_heaven_client_error(exc)