perf(db): add chat message range index

ddl/migrations/0220_chat_message_chat_created_idx.sql

@@ -0,0 +1,10 @@
+-- Chat unread/message fanout queries constrain by chat_id and created_at.
+-- The old partial index led with created_at and had 0 scans in production,
+-- while chat_message lookups were producing heavy temp I/O. Lead with chat_id
+-- so the per-chat range checks can use a small ordered slice.
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS chat_message_chat_created_non_blast_idx
+    ON public.chat_message USING btree (chat_id, created_at, user_id)
+    WHERE blast_id IS NULL;
+
+DROP INDEX CONCURRENTLY IF EXISTS public.chat_message_non_blast_created_at_idx;