Add admin buffer, dashboard, and item graph APIs

[starboyate]

Summary

• Add admin buffer APIs for listing conversation and insight buffers, inspecting detail, and applying scoped corrections.
• Add dashboard aggregation APIs for persisted memory, buffer, thread, and item graph health.
• Add item graph admin APIs for inspecting entities, aliases, mentions, links, batches, and applying targeted corrections.
• Flush insight construction before retrieval in the Java insight example.

Test Plan

• mvn test
• git diff --check origin/main...HEAD
• Secret keyword scan across the PR diff