Add backend documentation and initial database schema setup

Commit notes

- Created a README.md file for the backend, detailing the Bun + Elysia API setup, database configuration, and migration instructions. - Added initial SQL migration scripts for database schema, including the creation of essential tables and types for analytics and audio management. - Introduced metadata journal and snapshot files for tracking database migrations. - Updated Redis upload limits to enhance concurrent upload capabilities.

This commit establishes foundational documentation and database structure for the backend, improving developer onboarding and system organization.