feat(handlers): add a message handler

Signed-off-by: Sphericalkat <me@kat.bio>

main.py

@@ -1,7 +1,13 @@
 import logging
 
 from telegram import Update
-from telegram.ext import ContextTypes, ApplicationBuilder, CommandHandler
+from telegram.ext import (
+    ContextTypes,
+    ApplicationBuilder,
+    CommandHandler,
+    MessageHandler,
+    filters,
+)
 
 from settings import Settings
 
@@ -20,9 +26,19 @@ async def start(update: Update, context: ContextTypes.DEFAULT_TYPE):
     )
 
 
+async def message_handler(update: Update, context: ContextTypes.DEFAULT_TYPE):
+    await context.bot.send_message(
+        chat_id=update.effective_chat.id,
+        text="I'm sorry, I'm a bot and I can't understand that.",
+    )
+
+
 if __name__ == "__main__":
     app = ApplicationBuilder().token(settings.bot_token).build()
     start_handler = CommandHandler("start", start)
+    message_handler = MessageHandler(filters.ALL, message_handler)
+
     app.add_handler(start_handler)
+    app.add_handler(message_handler)
 
     app.run_polling()