add full-screen gift effects

apps/server-rust/migrations/009_gift_effect.sql

@@ -0,0 +1,7 @@
+-- Every account owns exactly one built-in full-screen gift-effect component.
+-- Existing accounts are backfilled from the validated Rust defaults during
+-- startup; the partial index keeps concurrent starts idempotent.
+
+CREATE UNIQUE INDEX IF NOT EXISTS component_instances_single_gift_effect
+  ON component_instances(owner_user_id, kind)
+  WHERE kind = 'gift_effect';