Set up backend for storing time capsules

[Kroszborg]

Backend Storage for Capsules

Issue Title: Set up backend for storing time capsules

Description: Create the logic and database schema for storing user-created time capsules with their reveal date.

Tasks:
    - Set up a database (SQL/NoSQL) for storing capsule metadata (user, reveal date, file links).
    - Write logic to store the uploaded files in cloud storage (e.g., AWS S3, Firebase).
    - Ensure files are linked to the correct user and reveal date.