[Broadcast] Delivery system

[viganogabriele]

Parent Epic: #120

Part of #120

Implement the actual message delivery to groups.

Requirements

• Fetch all groups belonging to the selected tag
• Send the formatted message to each group
• Track delivery status per group:
  • Sent successfully
  • Failed (reason: bot not in group, permission denied, rate limited, etc.)
• Handle errors gracefully:
  • Log failures
  • Continue with remaining groups
  • Update announcement status
• Rate limiting to avoid Telegram API blocks
• Generate delivery summary report:
  • Total groups
  • Success count
  • Failure count
  • Per-group status

Technical Notes

• Re-send failed messages (optional, configurable)
• Handle media upload efficiently (reuse file_id when possible)

Labels: enhancement