docs: Update CONTRIBUTING.md with database migration guidelines

[DylanDevelops]

Expanded the documentation to include instructions for adding and managing database migrations in the project. Clarified the structure of the database schema, added details about UTC timestamp storage, and updated the description of the internal storage module to mention migrations.

Pull Request Checklist

• I have read and followed the contribution guidelines.
• My pull request targets the main branch of tmpo.
• I have tested these changes locally on my machine.

Description

This pull request updates the documentation in CONTRIBUTING.md to clarify the structure and management of the database, with a particular focus on migrations and schema details. The changes provide clearer guidance for contributors on how to handle database schema modifications and describe the current schema in more detail.

Database schema and storage documentation:

• Expanded the description of the internal/storage/ directory to include migrations, not just database operations and models.
• Updated the database schema section to explicitly list the time_entries, milestones, and settings tables, including their purposes and a note about UTC timestamp storage and timezone conversion.

Guidance for database migrations:

• Added a new section with step-by-step instructions for creating and registering database migrations in internal/storage/migrations.go, including code examples and important safety notes about transactional execution.