Add Langfuse as a self-hosted eval and trace viewer

apps/ai-service/migrations/006_rag_response_payload.sql

@@ -0,0 +1,7 @@
+-- Full answer content for one trace row (answer text, blocks, citations,
+-- quick replies, plan, candidate assessments, disclaimer) — everything a
+-- client needs to redraw the assistant turn without re-running the query.
+-- Nullable/additive: existing rows simply have no replayable answer, and
+-- `/v1/rag/transcript` degrades to showing just the user turn for those.
+ALTER TABLE rag_retrieval_trace
+    ADD COLUMN IF NOT EXISTS response_payload jsonb;