feat(tgbot): route admin notifications to a forum topic (#6879)

Admins who run the bot in a forum supergroup got every notification in the
General topic. An admin chat ID may now carry a topic, chatId:topicId, and the
bot's admin messages and reports go to that topic. Repeated chats collapse into
one entry and a second, different topic for the same chat is rejected, in the
panel and in the docs setup helper alike. The chat ID hint in every locale and
the English, Persian, Russian and Chinese guides describe the form.

internal/web/translation/en-US.json

@@ -1343,7 +1343,7 @@
       "telegramAPIServer": "Telegram API Server",
       "telegramAPIServerDesc": "The Telegram API server to use. Leave blank to use the default server.",
       "telegramChatId": "Admin Chat ID",
-      "telegramChatIdDesc": "The Telegram Admin Chat ID(s). (comma-separated)(get it here {'@'}userinfobot) or (use '/id' command in the bot)",
+      "telegramChatIdDesc": "The Telegram Admin Chat ID(s). (comma-separated)(get it here {'@'}userinfobot) or (use '/id' command in the bot). To post into a forum topic, append ':topicId', e.g. -1001234567890:42",
       "telegramNotifyTime": "Notification Time",
       "telegramNotifyTimeDesc": "How often the Telegram bot sends periodic reports. Pick a preset interval, or choose Custom to enter a raw crontab expression.",
       "notifyTime": {