feat(api,userbot,frontend): fetch only new messages in backfill by default

backend/src/userbot/modules/capture/repository.py

@@ -118,6 +118,16 @@ async def upsert_message(  # noqa: PLR0913
     )
 
 
+async def max_message_id(
+    pool: asyncpg.Pool, account_id: int, chat_id: int
+) -> int | None:
+    return await pool.fetchval(
+        "SELECT max(message_id) FROM messages WHERE account_id = $1 AND chat_id = $2",
+        account_id,
+        chat_id,
+    )
+
+
 async def mark_deleted_box(
     pool: asyncpg.Pool, account_id: int, message_ids: list[int]
 ) -> None: