feat: persist queue probe metadata

server/migrations/0003_queue_probe_metadata.sql

@@ -0,0 +1,5 @@
+-- Persist only server-derived placement metadata alongside queue ownership.
+-- Redis remains a rebuildable index; this JSON projection is durable source
+-- data and may be empty until the authenticated probe completes.
+ALTER TABLE queue_tickets
+    ADD COLUMN predicted_rtt JSONB NOT NULL DEFAULT '{}'::jsonb;