Add Vercel deployment support with Flask webhook and cron-based reminders(To be accepted after the db migration PR)

[Jeyididya]

This PR adds support for deploying the bot on Vercel using a Flask webhook server and replaces the runtime JobQueue reminder system with a Vercel cron job. This allows the bot to run reliably on serverless infrastructure where long-running processes are not supported.

Key Changes

1. Flask webhook server
   Introduced a web_app.py Flask application to handle incoming Telegram webhook requests.
   Updates are processed asynchronously using the existing build_application logic.

2. Vercel deployment configuration
   Added vercel.json configuration for deploying the Python serverless function.

3. Cron-based reminder system
   Implemented /cron endpoint that triggers vercel_cron_reminders.
   Added Vercel cron configuration to run the reminder job daily.

4. Documentation
   Added deployment instructions for Vercel including:
   Environment variables setup
   Webhook configuration
   Cron job configuration

Why this change

Vercel’s serverless environment does not support persistent background workers like JobQueue.
This implementation enables the bot to run fully on Vercel while maintaining the reminder functionality through scheduled cron triggers.

Benefits
Enables serverless deployment
Removes dependency on persistent worker processes
Makes the bot easier to deploy and maintain
Keeps reminder functionality intact through scheduled checks

Reminder: Only to be accepted after the DB to SQLite migration PR

[vercel]

@Jeyididya is attempting to deploy a commit to the Amira's projects Team on Vercel.

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