Add index on "user".created_at

Adds migration 119 creating idx_user_created_at on "user"(created_at)
using CREATE INDEX CONCURRENTLY, matching the repo convention for
index-only migrations (114/115).

Co-authored-by: multica-agent <github@multica.ai>

server/migrations/119_user_created_at_index.down.sql

@@ -0,0 +1 @@
+DROP INDEX CONCURRENTLY IF EXISTS idx_user_created_at;

server/migrations/119_user_created_at_index.up.sql

@@ -0,0 +1,8 @@
+-- Index on "user".created_at to support ordering/filtering users by signup time.
+--
+-- "user" is a reserved word so it stays double-quoted. The migration runner
+-- executes files outside an explicit transaction, so CONCURRENTLY is kept in
+-- its own single-statement migration to avoid Postgres' implicit transaction
+-- block for multi-statement query strings.
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_user_created_at
+    ON "user" (created_at);