fix: add unique constraint to opportunity's slack message 👖

[ramiAbdou]

Description ✏️

This PR fixes an issue that sometimes duplicates an opportunity from a failed Bull job by adding a unique constraint on (slack_channel_id, slack_message_id).

Type of Change 🐞

• Feature - A non-breaking change which adds functionality.
• Fix - A non-breaking change which fixes an issue.
• Refactor - A change that neither fixes a bug nor adds a feature.
• Documentation - A change only to in-code or markdown documentation.
• Tests - A change that adds missing unit/integration tests.
• Chore - A change that is likely none of the above.

Checklist ✅

• I have done a self-review of my code.
• I have manually tested my code (if applicable).
• I have added/updated any relevant documentation (if applicable).