Add verbose request logs page

[BenItBuhner]

Summary

• Add a /logs admin page that lists recent requests and loads the selected request's verbose event trace.
• Reuse the existing event timeline to surface routing, fallback, key, proxy, error, and enforcement events.
• Enrich route trace events with masked API-key source and egress-proxy metadata, plus a dedicated proxy cooldown event.
• Add backend coverage for key/proxy metadata in admin event traces.

Testing

• bun test tests/admin-events.test.ts
• cd web && bun run typecheck
• bun run typecheck currently fails on base-branch errors unrelated to this PR:
  • src/server/routes/openai.ts extraHeaders not in EnforceCallArgs
  • tests/models-list.test.ts Bun fetch typing / RequestInfo issues