feat: RAG pipeline analytics

[jaikoo]

Summary

• Adds GET /v1/llm/rag endpoint for RAG pipeline analytics over retrieval span metadata (rag.* convention)
• New dashboard RAG tab with stat cards (total retrievals, avg relevance, context utilization), source table with latency percentiles, and relevance distribution bar
• Trace detail view shows inline RAG metadata on retrieval spans
• Migration 021 adds span_type index for efficient retrieval span filtering
• 3 new integration tests, E2E seed data with RAG trace, and spec updates

Test plan

• cargo test --features llm-tracing — 153 tests pass (48 LLM tracing including 3 new RAG tests)
• Manual: ingest trace with rag.* metadata, verify /v1/llm/rag returns populated response
• Manual: dashboard RAG tab renders stat cards and source table
• E2E: RAG tab appears in tab list and renders content