Is your feature request related to a problem?

We're in the middle of refactoring how plugin-server works internally and want to move action matching and other asynchronous processing into a separate process, reading from our clickhouse kafka events topic.

One piece of data that event matching specifically requires which we don't have access to is site_url, which is captured during initial request processing and (currently) passed all the way through ingestion.

site_url is used to provide linking capabilities to webhooks triggered from plugins.

Describe the solution you'd like

• Make SITE_URL a constance setting
• Expose it in instance settings pages
• Populate and keep track of it in plugin-server. Read value at boot and update when value changes.

Note that this is slightly complicated by poor choice of library - constance uses pythons pickle module to encode values. We should side-step that somehow. We could consider changing the library (or roll our own).

Note that if the env var is set, this overrides the constance setting.

Describe alternatives you've considered

...

Additional context

Related: #9737

cc @yakkomajuri - probably a great task to pick up around sprint goal. Note I have a WIP branch replacing constance from ~6 months ago.

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