Add Oban for background job processing

[elias-ba]

Summary

Set up Oban for reliable background job processing.

Use Cases

• Email notifications
• Slack notifications
• GitHub API sync
• Deadline reminders
• Scheduled sprint actions
• Webhook processing

Setup

```elixir

mix.exs

{:oban, "~> 2.17"}

config/config.exs

config :estim, Oban,
repo: Estim.Repo,
queues: [
default: 10,
notifications: 20,
github_sync: 5,
slack: 10
],
plugins: [
Oban.Plugins.Pruner,
{Oban.Plugins.Cron, crontab: [
{"0 9 * * 1-5", Estim.Workers.DeadlineReminder},
]}
]
```

Job Modules

• `NotificationWorker` - Email/Slack notifications
• `GitHubSyncWorker` - Sync issues from GitHub
• `DeadlineReminderWorker` - Sprint deadline reminders
• `WebhookWorker` - Process incoming webhooks

Monitoring

• Add Oban Web dashboard (optional)
• Set up error alerting
• Configure job retention

Acceptance Criteria

• Oban configured and running
• At least one job type working
• Failed jobs retry appropriately
• Old jobs pruned automatically