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

backend/src/api/routers/backfill.py

@@ -16,6 +16,7 @@ class BackfillRequest(BaseModel):
     account_id: int
     chat_id: int
     media: bool = False
+    full: bool = False
 
 
 class FetchMediaRequest(BaseModel):
@@ -70,7 +71,7 @@ async def enqueue_backfill(
         pool,
         body.account_id,
         "backfill",
-        {"chat_id": body.chat_id, "media": body.media},
+        {"chat_id": body.chat_id, "media": body.media, "full": body.full},
     )
     return EnqueueResponse(job_id=job_id)