Pick up new channels/threads in known guilds without a restart

on_message only matched channels enumerated during the backlog scan,
so a brand-new channel (or any type not scanned, like threads) was
silently ignored until the next restart. Now falls back to a
guild-level lookup and registers the channel for live watching going
forward the first time a message arrives there.

main.py

@@ -27,6 +27,7 @@ class ReactorClient(discord.Client):
         self.display = display
         self.undo = undo
         self.known_channel_targets: dict[int, Target] = {}
+        self.known_guild_targets: dict[int, Target] = {}
         self._started = False
 
     async def on_ready(self):
@@ -92,6 +93,7 @@ class ReactorClient(discord.Client):
                     cutoff, react_fn, display=self.display,
                 )
             else:
+                self.known_guild_targets[target.target_id] = target
                 for channel in get_scannable_channels(self, target.target_id):
                     self.known_channel_targets[channel.id] = target
                     await scan_channel_backlog(
@@ -107,6 +109,20 @@ class ReactorClient(discord.Client):
         if self.undo:
             return
         target = self.known_channel_targets.get(message.channel.id)
+        if target is None:
+            # Channel didn't exist (or wasn't enumerated, e.g. a thread) at
+            # backlog-scan time. If it belongs to an already-known mutual
+            # guild, start watching it now instead of requiring a restart —
+            # only its live messages going forward are covered, no backfill.
+            guild = getattr(message, "guild", None)
+            if guild is not None:
+                target = self.known_guild_targets.get(guild.id)
+                if target is not None:
+                    self.known_channel_targets[message.channel.id] = target
+                    log.info(
+                        "New channel #%s in %s wasn't seen during backlog scan; watching it live from now on",
+                        getattr(message.channel, "name", message.channel.id), target.name,
+                    )
         if target is None:
             return
         if message.author.id != self.cfg.friend_id: