feat: attachment support for Discord and TUI channels #2002
Description
Epic: #1978
Only CLI and Telegram channels handle attachments (images). Discord and TUI silently drop them.
Acceptance Criteria:
- Discord channel handles image attachments from messages
- TUI channel supports image display (ratatui image widget or path display)
- All channel tests pass
Complexity: Medium | Effort: 4h
Ref: CHAN-03