 Implement Payment Comments and Notes

**Type**: Feature  
**Priority**: Medium  
**Labels**: `feature`, `backend`, `collaboration`

**Description**:
Allow users to add comments and notes to payments.

**Features**:
- Comment threads
- @mentions
- File attachments
- Comment notifications
- Comment history

**Acceptance Criteria**:
- Comments working
- Notifications sent
- Files can be attached
- All CI/CD checks must pass

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