feat：新增大屏存取发布接口

app/services/thumbnails.py

@@ -0,0 +1,63 @@
+"""大屏封面图：放本地磁盘，文件名就是封面 ID；每次上传换新 ID，旧图删掉
+
+图片类 URL 要能被 <img src> 匿名访问，靠不可猜的 ID 兜底
+"""
+
+import re
+import secrets
+from pathlib import Path
+
+from app.core.exceptions import BizException
+
+MAX_BYTES = 1024 * 1024
+# 按文件头判断类型，不信任客户端声明的 Content-Type
+MAGIC = {b"\x89PNG\r\n\x1a\n": "image/png", b"\xff\xd8\xff": "image/jpeg"}
+# ID 进了文件路径，读写前必须过这个格式检查，防路径穿越
+ID_PATTERN = re.compile(r"th_[A-Za-z0-9_-]{16,64}")
+
+
+def sniff(head: bytes) -> str | None:
+    for magic, media_type in MAGIC.items():
+        if head.startswith(magic):
+            return media_type
+    return None
+
+
+class ThumbnailStorage:
+    def __init__(self, root: Path):
+        self._root = root
+
+    def _path(self, thumbnail_id: str) -> Path | None:
+        if not ID_PATTERN.fullmatch(thumbnail_id):
+            return None
+        return self._root / thumbnail_id
+
+    def save(self, data: bytes) -> str:
+        if len(data) > MAX_BYTES:
+            raise BizException(413, "封面图不能超过 1 MB")
+        if sniff(data[:8]) is None:
+            raise BizException(422, "封面图只支持 PNG 或 JPEG")
+        thumbnail_id = f"th_{secrets.token_urlsafe(16)}"
+        self._root.mkdir(parents=True, exist_ok=True)
+        self._path(thumbnail_id).write_bytes(data)
+        return thumbnail_id
+
+    def open(self, thumbnail_id: str) -> tuple[Path, str] | None:
+        """文件路径和图片类型；ID 不合法、文件不存在或内容不是图片时返回 None"""
+        path = self._path(thumbnail_id)
+        if path is None or not path.is_file():
+            return None
+        with path.open("rb") as file:
+            media_type = sniff(file.read(8))
+        return None if media_type is None else (path, media_type)
+
+    def copy(self, thumbnail_id: str) -> str | None:
+        path = self._path(thumbnail_id)
+        if path is None or not path.is_file():
+            return None
+        return self.save(path.read_bytes())
+
+    def delete(self, thumbnail_id: str | None) -> None:
+        path = None if thumbnail_id is None else self._path(thumbnail_id)
+        if path is not None:
+            path.unlink(missing_ok=True)