[Broadcast] Dashboard views for announcements #28

Parent Epic: #26

Part of: PoliNetworkOrg/telegram#120

Implement dashboard views for managing announcements.

Requirements

Scheduled Announcements List

• View all scheduled announcements:
  • Status (pending, sent, failed, cancelled)
  • Tag number
  • Target groups count
  • Scheduled datetime
  • Message preview (text + thumbnail if media)
  • Creator admin

Modify Pending Announcements

• Edit target groups (select different tag)
• Edit message content (text and media)
• Reschedule delivery time
• Cancel announcement
• Change from scheduled to immediate

Delivery Reports

• View delivery results:
  • Success/failure statistics
  • Per-group delivery status
  • Error details for failed groups
• Filter by date range, status, tag

API Endpoints (backend)

• • list with filters
• • details
• • modify pending
• • cancel

Labels: enhancement