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/knowledge.py

@@ -2,12 +2,23 @@ from __future__ import annotations
 
 import json
 from functools import lru_cache
+from pathlib import Path
 from typing import Any
 
 from backend.bootstrap.config import APP_DIR
 
 
 KNOWLEDGE_FILE = APP_DIR / "data" / "heaven_knowledge.json"
+# Baked into the image outside the ./data bind mount so volume overlay cannot hide it.
+KNOWLEDGE_SEED_FILE = Path(__file__).resolve().parent / "assets" / "heaven_knowledge.json"
+
+
+class HeavenKnowledgeError(ValueError):
+    """Structured knowledge-file failure surfaced to HTTP as Chinese API errors."""
+
+    def __init__(self, message: str, *, code: str) -> None:
+        super().__init__(message)
+        self.error_code = code
 
 
 def prepare_heaven_context(mode: str, calculation: dict[str, Any]) -> dict[str, Any]:
@@ -379,9 +390,49 @@ def _line_record(line: dict[str, Any]) -> dict[str, Any]:
     }
 
 
+def resolve_heaven_knowledge_path() -> Path:
+    """Prefer the persisted data-dir file; fall back to the image-baked seed."""
+    if KNOWLEDGE_FILE.is_file():
+        return KNOWLEDGE_FILE
+    if KNOWLEDGE_SEED_FILE.is_file():
+        return KNOWLEDGE_SEED_FILE
+    raise HeavenKnowledgeError(
+        "问天知识文件缺失：未找到 heaven_knowledge.json。"
+        "请确认宿主机 data 目录或镜像内 seed 文件完整。",
+        code="heaven_knowledge_missing",
+    )
+
+
 @lru_cache(maxsize=1)
 def _knowledge_catalog() -> dict[str, Any]:
-    payload = json.loads(KNOWLEDGE_FILE.read_text(encoding="utf-8"))
+    path = resolve_heaven_knowledge_path()
+    try:
+        raw = path.read_text(encoding="utf-8")
+    except OSError as exc:
+        raise HeavenKnowledgeError(
+            f"问天知识文件无法读取（{path.name}）：{exc.strerror or exc}",
+            code="heaven_knowledge_missing",
+        ) from exc
+    try:
+        payload = json.loads(raw)
+    except json.JSONDecodeError as exc:
+        raise HeavenKnowledgeError(
+            f"问天知识文件 JSON 损坏（{path.name}），无法解析："
+            f"第 {exc.lineno} 行附近。",
+            code="heaven_knowledge_invalid",
+        ) from exc
+    if not isinstance(payload, dict):
+        raise HeavenKnowledgeError(
+            f"问天知识文件格式不正确（{path.name}）：根节点必须是对象。",
+            code="heaven_knowledge_invalid",
+        )
     if not payload.get("version") or not isinstance(payload.get("sources"), dict):
-        raise ValueError("问天知识库格式不完整。")
+        raise HeavenKnowledgeError(
+            f"问天知识库格式不完整（{path.name}）：缺少 version 或 sources。",
+            code="heaven_knowledge_invalid",
+        )
     return payload
+
+
+def clear_heaven_knowledge_cache() -> None:
+    _knowledge_catalog.cache_clear()