AI 配置改为按用户独立存储

每个用户 AI 配置存 data/user_data/ai_settings/{username}.json，
读配置时用户级优先，无则回退到全局 secrets.json。

Co-Authored-By: Claude <noreply@anthropic.com>

serve/backend/app/secrets_store.py

@@ -94,3 +94,53 @@ def mask(key: str, prefix: int = 4, suffix: int = 4) -> str:
     if len(key) <= prefix + suffix:
         return "•" * len(key)
     return f"{key[:prefix]}{'•' * 6}{key[-suffix:]}"
+
+
+# ================================================================
+# 按用户 AI 配置 (data/user_data/ai_settings/{username}.json)
+# ================================================================
+
+def _ai_config_path(username: str) -> Path:
+    from app.config import settings
+    p = settings.data_dir / "user_data" / "ai_settings" / f"{username}.json"
+    p.parent.mkdir(parents=True, exist_ok=True)
+    return p
+
+
+def load_ai_config(username: str) -> dict:
+    """加载指定用户的 AI 配置。用户不存在时返回空 dict。"""
+    p = _ai_config_path(username)
+    if p.exists():
+        try:
+            return json.loads(p.read_text(encoding="utf-8"))
+        except Exception as e:  # noqa: BLE001
+            logger.warning("ai_settings/%s.json malformed: %s", username, e)
+    return {}
+
+
+def save_ai_config(username: str, updates: dict) -> dict:
+    """合并写入指定用户的 AI 配置。返回新内容。"""
+    current = load_ai_config(username)
+    current.update({k: v for k, v in updates.items() if v is not None})
+    p = _ai_config_path(username)
+    p.write_text(json.dumps(current, indent=2, ensure_ascii=False), encoding="utf-8")
+    try:
+        os.chmod(p, 0o600)
+    except OSError:
+        pass
+    return current
+
+
+def clear_ai_config(username: str, *keys: str) -> dict:
+    """清掉指定用户的 AI 配置字段。不传 keys 则删除整个文件。"""
+    p = _ai_config_path(username)
+    if not p.exists():
+        return {}
+    if not keys:
+        p.unlink()
+        return {}
+    current = load_ai_config(username)
+    for k in keys:
+        current.pop(k, None)
+    p.write_text(json.dumps(current, indent=2, ensure_ascii=False), encoding="utf-8")
+    return current