fix(conversations): preserve two-member groups

[rissrice2105-agent]

Summary

• classify global conversation deletion only from the stored direct type
• keep group deletion user-scoped even when a group has exactly two participants
• add a regression that models a two-member group and verifies the service-role deletion path is not used

Closes #233

Testing

• focused Vitest route regression — 1 passed
• targeted ESLint with zero warnings
• node --check on the changed route and test
• git diff --check