Wire up query history: localStorage session persistence + sidebar UI

apps/ai-service/migrations/005_rag_trace_conversation.sql

@@ -0,0 +1,6 @@
+ALTER TABLE rag_retrieval_trace
+    ADD COLUMN IF NOT EXISTS conversation_id varchar(128);
+
+CREATE INDEX IF NOT EXISTS rag_retrieval_trace_conversation_idx
+    ON rag_retrieval_trace (conversation_id, created_at DESC)
+    WHERE conversation_id IS NOT NULL;