Queue static cache gateway purges

[timkelty]

Summary

• dispatch saved-element gateway purge requests through a Craft queue job instead of calling the gateway inline
• preserve final event-mutated tags, deduplicated fetchUrls, delete/header behavior, and Cache-Purge-Tag fallback when queue dispatch fails
• keep non-web gateway purges synchronous for backward compatibility and document the queued behavior

This is a stacked follow-up to #188; it does not change that pull request.

Validation

• composer test (77 tests, 232 assertions, 1 existing Craft 4-only skip)
• composer phpstan
• composer cs:check
• git diff --check