feat(4.0-P7): 签名目录来源：LLM 安装器泛化为通用插件安装器

- SignedCatalogInstaller 提供目录 / 插件包验签、sha256、防 zip-slip 与落盘；PluginInstaller 从 plugins tag 安装 plugin.toml 插件，LLM 安装器改为其子类，行为不变
- 加载器新增 catalog 来源（builtin > catalog > local > pip），视为已签名
- 新增 GET /plugins/catalog、POST /plugins/{id}/install、DELETE /plugins/{id}；安装前拒绝保留 id、非法 id 与内置同名，卸载只删除安装器装入的目录
- scripts/build_plugin_catalog.py 生成并签名 catalog.json

scripts/build_plugin_catalog.py

@@ -0,0 +1,77 @@
+"""生成并签名插件目录（``plugins`` tag 的发布产物）。
+
+用法（需要更新签名私钥，见 CLAUDE.md 的发布说明）::
+
+    uv run --no-project --with cryptography python scripts/build_plugin_catalog.py \
+        --key ~/.autobangumi/update-signing-key.pem --min-ab 4.0.0 \
+        --out release-assets  dist/*.zip
+
+输入是 ``ab-plugin pack`` 打出的 zip。输出目录里有 ``catalog.json``、每个 zip 以及
+它们的 ``.sig``（对文件全部字节做 ed25519 签名，base64 文本），一并上传到 GitHub
+release ``plugins`` 即可。安装端的校验见 ``backend/src/module/plugin/installer.py``。
+"""
+
+import argparse
+import base64
+import hashlib
+import json
+import shutil
+import tomllib
+import zipfile
+from pathlib import Path
+
+from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
+from cryptography.hazmat.primitives.serialization import load_pem_private_key
+
+CATALOG_SCHEMA = 2
+
+
+def _entry(archive: Path, min_ab: str) -> dict:
+    with zipfile.ZipFile(archive) as zf:
+        plugin = tomllib.loads(zf.read("plugin.toml").decode("utf-8"))["plugin"]
+    return {
+        "id": plugin["id"],
+        "name": plugin["name"],
+        "version": plugin["version"],
+        "kind": "plugin",
+        "extension_points": plugin.get("extension_points", []),
+        "sdk": plugin["sdk"],
+        "min_ab_version": min_ab,
+        "description": plugin.get("description", ""),
+        "asset": archive.name,
+        "sha256": hashlib.sha256(archive.read_bytes()).hexdigest(),
+    }
+
+
+def _sign(key: Ed25519PrivateKey, path: Path) -> None:
+    signature = key.sign(path.read_bytes())
+    Path(str(path) + ".sig").write_text(base64.b64encode(signature).decode() + "\n")
+
+
+def build(archives: list[Path], out: Path, key_path: Path, min_ab: str) -> Path:
+    key = load_pem_private_key(key_path.read_bytes(), password=None)
+    if not isinstance(key, Ed25519PrivateKey):
+        raise SystemExit("签名密钥必须是 ed25519")
+    out.mkdir(parents=True, exist_ok=True)
+    entries = []
+    for archive in sorted(archives):
+        entries.append(_entry(archive, min_ab))
+        shutil.copy2(archive, out / archive.name)
+        _sign(key, out / archive.name)
+    catalog = out / "catalog.json"
+    catalog.write_text(
+        json.dumps({"schema": CATALOG_SCHEMA, "plugins": entries}, indent=2) + "\n",
+        encoding="utf-8",
+    )
+    _sign(key, catalog)
+    return catalog
+
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser(description=__doc__.splitlines()[0])
+    parser.add_argument("archives", nargs="+", type=Path)
+    parser.add_argument("--key", required=True, type=Path)
+    parser.add_argument("--out", required=True, type=Path)
+    parser.add_argument("--min-ab", default="4.0.0")
+    args = parser.parse_args()
+    print(build(args.archives, args.out, args.key, args.min_ab))