Two-phase backlog scan with self-healing found-count tracking

Split each channel's backlog scan into a find phase (settles the
found count first) and a react phase, so the progress display no
longer grows both numbers in lockstep. Found messages are logged in
a dedicated, message_id-deduped table and recovered across restarts
independent of the resume checkpoint, so nothing found is silently
abandoned if the script stops mid-react.

Also drops the separately-maintained messages_found counter, which
proved prone to drift under repeated interruptions/rescans, in favor
of computing it live from the dedup table - self-healing regardless
of how many times a channel gets rescanned.

bot/reactor.py

@@ -26,10 +26,18 @@ async def react_to_message(
     emoji: str,
     source: str,
     display: Optional[StatsDisplay] = None,
+    count_found: bool = True,
 ) -> bool:
     if message.author.id != friend_id:
         return False
 
+    if count_found:
+        await asyncio.to_thread(
+            storage.record_message_found, message.id, message.channel.id, target.target_id, friend_id
+        )
+        if display is not None:
+            await display.refresh()
+
     already_reacted = any(
         reaction.me and str(reaction.emoji) == emoji for reaction in message.reactions
     )