T3096 Refactor letters display

[ecino]

• ADD new field for tracking date of Exit communication sent to sponsor
• FIX is_final_letter field

[gemini-code-assist]

Code Review

This pull request introduces a new exit_communication_sent field to track when exit communications are sent to partners, including a post-migration script to backfill existing data. It also refactors the final letter logic and updates the correspondence tree view decoration. Feedback focuses on the migration script, highlighting a logic error in the SQL query's operator precedence, potential false positives when matching IDs in strings, and several performance issues related to database operations inside loops. Additionally, it is recommended to use UTC time for consistency with Odoo's standards.