Scheduled announcements — /announce command

[BillChirico]

Part of #40

Allow admins to schedule one-time or recurring messages via /announce.

Subtasks

• Create scheduled_messages table: id, guild_id, channel_id, content, embed_json, cron_expression, next_run, timezone, author_id, enabled, created_at
• /announce once — schedule a one-time message at a specific datetime
• /announce recurring — schedule a recurring message (daily, weekly, etc.)
• /announce list — show all scheduled messages for the guild with next fire time
• /announce cancel — cancel a scheduled message (author or admin)
• Lightweight scheduler: setInterval polling every 60s, checks next_run, fires and updates
• Support rich embeds via optional --embed flag with title/description/color
• Timezone support via guild-level config (default America/New_York)

Acceptance Criteria

• One-time and recurring announcements work
• Scheduler persists across restarts
• Timezone-aware scheduling