Protect Algolia documentation API keys

[fsfh60]

Summary

• protect Algolia documentation tool API keys before storing them
• keep existing Algolia keys when edit forms are saved blank and unprotect keys at use time
• add regression coverage and changelog entry

Validation

• dotnet test ./tests/CrestApps.Core.Tests/CrestApps.Core.Tests.csproj -c Release /p:NuGetAudit=false /p:CopilotSkipCliDownload=true --filter DocumentationSearchTests
• npm run build --prefix ./src/CrestApps.Core.Docs