Endpoint for batch rule unsubscription

[bracyw]

Changes

Added POST /rules/unsubscribe endpoint for batch rule unsubscription. Takes rule_ids and client_id in request body.

Implementation removes subscriptions while keeping rules available for future re-subscription. O(n) time complexity as specified.

Test Cases

• Success case with partial unsubscription (rule remains if other clients subscribed)
• Orphaned rules remain available (not deleted)
• Orphaned rule resubscription
• Nonexistent rule handling (no-op)
• Empty list edge case

All tests pass.

Checklist

• All commits are tagged with the ticket number
• No linting errors / newline at end of file warnings
• All code follows repository-configured prettier formatting
• No merge conflicts
• All checks passing
• Remove any non-applicable sections of this template
• Request reviewers & ping on Slack
• PR is linked to the ticket (see below)

Closes #498