Switch DB from SQLite to Postgres.

[Jeyididya]

What this PR does:

• Replaces SQLite with PostgreSQL as the database backend.
• All existing table structures and methods are preserved.
• Uses CREATE TABLE IF NOT EXISTS and ALTER TABLE ... IF NOT EXISTS for safe initialization in serverless environments.
• Uses psycopg2 .
• Environment variables are used for DB credentials (DB_HOST, DB_NAME, DB_USER, DB_PASS, DB_PORT).
• Prepares the code for serverless hosting (e.g., Vercel cron functions) by making table initialization safe and efficient.

Why this change is needed:

• SQLite cannot be used reliably on serverless platforms like Vercel.
• Postgres provides persistent, cloud-hosted storage suitable for cron-based reminders and long-term scaling.

[vercel]

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

A member of the Team first needs to authorize it.