fix: harden the debug command, link cache and rate limiter

- commands: /bot_dict dumped the whole chat state to any member of the chat
  and could exceed Telegram's 4096-char message limit (the send then failed
  and bubbled up as a handler error). It is now admin-only and capped at
  MAX_DEBUG_DUMP_CHARS; README, README.en and the /help description updated.
- send: the edit-before-forward template buttons were built from a HashMap
  walk, so their order changed between prompts. Now sorted by name.
- link_cache: an unparseable payload (older schema) was reported as a miss
  but left in place, re-failing the parse on every later hit; the row is
  dropped on read.
- handlers: a link handed to the URL workers after the channel closed
  (shutdown) was discarded silently; it is now logged.
- rate_limit: LIMITERS kept one bucket per chat that ever sent media,
  forever. The periodic sweep now drops buckets that are idle (refilled to
  capacity) and not held by an in-flight sender; acquire's refill was
  factored into a shared helper used by the idle check.

Tests: +3 (corrupted row dropped, sorted markup, idle-bucket pruning); the
cache one was verified to fail before the fix. fmt/clippy clean, 55 + 69.

README.md

@@ -113,7 +113,7 @@ Telegram 只接受 443/80/88/8443 端口。
 | `/set_template <名称>` | 回复一条含 `[]` 的消息，将其保存为命名模板；转发时 `[]` 会被替换为原帖链接（配合「转发前编辑」使用） |
 | `/set_format <站点> <格式>` | 自定义某站点的 caption 格式。站点：`twitter` / `bsky` / `pixiv` / `misskey`。占位符：`{url}` `{author}` `{author_url}` `{title}` `{tags}` |
 | `/clear_cache [链接]` | 清空链接缓存（仅管理员）；带链接只清该条，否则清空全部 |
-| `/bot_dict` | 查看当前聊天状态（调试用） |
+| `/bot_dict` | 查看当前聊天状态（调试用；仅管理员） |
 | `/test <链接>` | 调试：只解析链接并返回解析结果（站点、标题、作者、标签、媒体列表），不发送任何媒体 |
 
 链接处理仅限私聊；命令在任意聊天可用。