feat: support multiple webhook targets via alert_webhook_url array

[eclectic-coding]

Summary

• alert_webhook_url now accepts a string (existing behavior, unchanged) or an array of URL strings
• All configured endpoints receive the same JSON payload when the failure threshold is exceeded
• A failure posting to one URL is logged to Rails.logger and skipped without blocking the remaining targets
• Single-string configs require no changes — fully backward-compatible

Implementation

All changes are in AlertWebhook:

• webhook_urls private method normalizes the config value with Array(...).flatten.compact.select(&:present?)
• configured? checks webhook_urls.any? instead of .present?
• call captures the URL list before spawning the thread, then iterates in the thread

3 new specs: posts to all URLs, continues after one failure, treats an empty array as unconfigured.

Test plan

• Single string config — existing behavior unchanged; one POST fired
• Array config with two URLs — both endpoints receive the payload on the same alert
• One URL in the array returns an error — error logged, second URL still receives the payload
• Empty array — no POST fired

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