Add batch size option for webhooks

[rjnz2023]

Summary

When dealing with high event volumes, webhooks can end up with very large payloads that may overwhelm the receiving endpoint. This PR adds a configurable batch-size option that limits how many events are included in each webhook request.

Key changes:

• New batch-size config option under webhook.<id>.batch-size (defaults to 100)
• Events beyond the batch limit are queued for the next delivery cycle
• Invalid batch sizes trigger a warning and fall back to the default

Configuration

[webhook.my-webhook]
url = "https://example.com/webhook"
batch-size = 50  # Send max 50 events per request

Testing

• Verified compilation with cargo check
• Logic ensures remaining events are scheduled for delivery after throttle interval

There's a companion PR for the webadmin UI to expose this setting.

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✅ rjnz2023
❌ mdecimus
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