Adds /dashboard/decisions page with colored type badges, search,
filters (by type), sort (newest/oldest/costliest), pagination,
and links to parent traces. New /api/decisions endpoint with
Prisma queries. Removes 'Soon' badge from sidebar nav.
- LangChain: AgentLensCallbackHandler with auto-span creation for
LLM calls, tool calls, chains, and agent decision logging
- Dashboard: trace list with search, status filters, pagination
- Dashboard: trace detail with Decision/Span/Event tabs
- Dashboard: sidebar layout, responsive design, dark theme
