Expose more of the backend system config to the frontend

[JoshuaJewell]

Problem

Some existing backend system config needs to be mutable from the frontend: specifically, config/primers.yml, config/composition/ filter lists, and some keys from databases.yml (possibly for moving into pipeline.yml?).

Right now, system-level behaviour in these files requires manual config updates. The frontend should provide a controlled way to view and edit supported backend system config values.

Proposed behaviour

Add backend keys to the existing system config interface that can:

• fetch supported backend system config
• display editable config fields
• update mutable config values
• persist changes through the backend
• avoid requiring manual backend edits for user-scoped config changes