Add Web Vitals performance tracking (LCP, INP, CLS, FCP, TTFB)

End-to-end performance metrics tracking: dedicated database table with
percentile aggregation, tracker collection behind data-perf attribute,
/api/send handling, report API endpoints, and Performance dashboard page
with threshold-based metric cards, time-series chart, and per-page breakdown.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>