Add stored observation history and broker-scoped cache summaries

[Dumdart]

Summary

• Add an observation history tab with topic, date-time, ordering, and result-limit filters
• Render stored observation results and inspect payloads as plain text
• Add broker-scoped cache summaries while preserving global totals
• Refresh stored observation data after subscription changes when required

Testing

• Added GUI coverage for history rendering, payload inspection, accessible controls, and cache summaries
• Added view-model coverage for stored observation query filtering and threaded runtime access