feat: Implement file-based persistence for user registration

This commit introduces a file-based persistence layer to store and retrieve registered user chat IDs.

Previously, the bot used an in-memory `set` to track registered users, which was cleared on every restart. This caused the bot to lose track of all registered users, preventing them from accessing registered-only features.

This commit resolves the issue by:
- Creating a `data/registered_users.txt` file to store user chat IDs.
- Implementing a `load_registered_users` function to load the user data from the file at startup.
- Modifying the `register_user` function to append new user chat IDs to the file.
- Calling `load_registered_users` from `main.py` at startup to initialize the user state.

app/main.py

@@ -18,7 +18,10 @@ from telegram.ext import Application, Defaults, CommandHandler, ContextTypes
 # --- IMPORTAR HABILIDADES ---
 from modules.flow_builder import load_flows
 from modules.logger import log_request
-from modules.database import chat_id_exists  # Importar chat_id_exists
+from modules.database import (
+    chat_id_exists,
+    load_registered_users,
+)  # Importar chat_id_exists y load_registered_users
 from modules.ui import main_actions_keyboard
 
 from modules.rh_requests import vacaciones_handler, permiso_handler
@@ -106,6 +109,11 @@ async def menu_principal(update: Update, context: ContextTypes.DEFAULT_TYPE):
 
 
 async def post_init(application: Application):
+    """Tareas a ejecutar después de que el bot se haya inicializado."""
+    # Cargar usuarios registrados desde el archivo de persistencia.
+    # Es crucial que esto se ejecute antes de que el bot empiece a recibir updates.
+    load_registered_users()
+
     # Mantén los comandos rápidos disponibles en el menú de Telegram
     await application.bot.set_my_commands(
         [