Document that cluster credential changes require an API restart

[SoundMindsAI]

Summary

Cluster credentials are resolved through a cached Settings accessor, so changing a mounted secret doesn't take effect until the API process restarts. This isn't documented, so an operator who rotates a credential and sees no change has no signal that a restart is needed.

Fix

Add a short callout to the relevant runbook (Solr / cluster-registration credential handling) noting that credential changes require an API restart to take effect, and briefly why (Settings caching).

Definition of done

• A clear "requires API restart" note added to the appropriate runbook under docs/03_runbooks/
• Note explains the cause (cached Settings) in one sentence
• No code changes

Background

Captured at docs/00_overview/planned_features/02_mvp2/chore_solr_cred_backfill_needs_api_restart/idea.md.

Good first issue: docs-only; teaches how Settings caching affects operator workflows.