merge: week4 - production ready, caching, performance, polished UI

feat(ui): add live system health panel in sidebar

feat(ui): add 4-stat grid, clear cache button, fixed polling

feat(ui): add export chat, avg latency display, message count

feat(ui): add per-message latency badge and copy button

feat(ui): add relevance % coloring and numbered source chips

feat(frontend): add session persistence and latency tracking

feat(frontend): add clearCache, deleteChatSession, getSystemHealth

test: add Week 4 performance test with cache and concurrency checks

feat(admin): add 4-stat health, cache clear endpoint

feat(chat): add latency metrics, session delete endpoint

feat(upload): clear query cache after new document indexed

perf(retrieval): add 5-minute query cache with 100 entry limit

perf(server): add request timing middleware and system health endpoint

fix: correct document status handling in admin routes