feat(memory-gateway): 引入 Memory Gateway 配置、凭据存储和服务编排

* 新增 MemoryGatewayConfig 和 MemoryConfig dataclass，用于配置管理。
* 实现 MemoryGatewayUserCredential 和 MemoryGatewayCredentialStore，用于处理用户凭据。
* 创建 MemoryGatewayService，用于管理与 Memory Gateway 的交互。
* 开发用于记忆设置的 JSON 配置文件。
* 增强单元测试，覆盖新功能，包括凭据存储和服务行为。
* 更新 entrypoint 和实例创建脚本，以初始化 Memory Gateway 用户存储。

app-instance/backend/beaver/memory/gateway/credentials.py

@@ -0,0 +1,75 @@
+"""Per-instance credential storage for Memory Gateway users."""
+
+from __future__ import annotations
+
+import json
+import os
+import tempfile
+from dataclasses import dataclass, field
+from pathlib import Path
+from typing import Any
+
+
+@dataclass(slots=True)
+class MemoryGatewayUserCredential:
+    user_id: str
+    user_key: str = field(repr=False)
+
+
+class MemoryGatewayCredentialStore:
+    """Persist Beaver username -> Gateway credential mappings."""
+
+    def __init__(self, path: str | Path) -> None:
+        self.path = Path(path)
+
+    def get(self, username: str) -> MemoryGatewayUserCredential | None:
+        users = self._load_users()
+        payload = users.get(username)
+        if not isinstance(payload, dict):
+            return None
+        user_id = str(payload.get("userId") or "").strip()
+        user_key = str(payload.get("userKey") or "").strip()
+        if not user_id or not user_key:
+            return None
+        return MemoryGatewayUserCredential(user_id=user_id, user_key=user_key)
+
+    def save(self, username: str, credential: MemoryGatewayUserCredential) -> None:
+        self.path.parent.mkdir(parents=True, exist_ok=True)
+        users = self._load_users()
+        users[username] = {
+            "userId": credential.user_id,
+            "userKey": credential.user_key,
+        }
+        payload = {"users": dict(sorted(users.items()))}
+        fd, tmp_name = tempfile.mkstemp(
+            prefix=f".{self.path.name}.",
+            suffix=".tmp",
+            dir=str(self.path.parent),
+        )
+        tmp_path = Path(tmp_name)
+        try:
+            with os.fdopen(fd, "w", encoding="utf-8") as handle:
+                json.dump(payload, handle, ensure_ascii=False, indent=2)
+                handle.write("\n")
+            os.chmod(tmp_path, 0o600)
+            os.replace(tmp_path, self.path)
+            os.chmod(self.path, 0o600)
+        finally:
+            if tmp_path.exists():
+                tmp_path.unlink()
+
+    def _load_users(self) -> dict[str, Any]:
+        if not self.path.exists():
+            return {}
+        data = json.loads(self.path.read_text(encoding="utf-8"))
+        if not isinstance(data, dict):
+            return {}
+        users = data.get("users")
+        return users if isinstance(users, dict) else {}
+
+
+def default_memory_gateway_users_path() -> Path:
+    raw = os.getenv("BEAVER_MEMORY_GATEWAY_USERS_PATH")
+    if raw:
+        return Path(raw)
+    return Path.home() / ".beaver" / "memory_gateway_users.json"