Add support for backfilling private chats

mautrix_telegram/commands/telegram/misc.py

@@ -315,5 +315,10 @@ async def backfill(evt: CommandEvent) -> None:
     try:
         await portal.backfill(evt.sender)
     except TakeoutInitDelayError:
-        await evt.reply("Please accept the data export request from a mobile device, "
-                        "then re-run the backfill command.")
+        msg = ("Please accept the data export request from a mobile device, "
+               "then re-run the backfill command.")
+        if portal.peer_type == "user":
+            from mautrix.appservice import IntentAPI
+            await portal.main_intent.send_notice(evt.room_id, msg)
+        else:
+            await evt.reply(msg)

mautrix_telegram/portal/telegram.py

@@ -361,6 +361,11 @@ class PortalTelegram(BasePortal, ABC):
         min_id = last.tgid if last else 0
         self.backfilling = True
         self.backfill_leave = set()
+        if self.peer_type == "user":
+            sender = p.Puppet.get(source.tgid)
+            await self.main_intent.invite_user(self.mxid, sender.default_mxid)
+            await sender.default_mxid_intent.join_room_by_id(self.mxid)
+            self.backfill_leave.add(sender.default_mxid_intent)
         max_file_size = min(config["bridge.max_document_size"], 1500) * 1024 * 1024
         async with source.client.takeout(files=True, megagroups=self.megagroup,
                                          chats=self.peer_type == "chat",