feat(backend): persist per-experiment plot_config via PUT /:id/plot-config

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/prisma/migrations/20260601000000_add_experiment_plot_config/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "experiments" ADD COLUMN     "plot_config" JSONB;

backend/prisma/schema.prisma

@@ -14,6 +14,7 @@ model Experiment {
   created_at            DateTime @default(now())
   template              Json     @default("[]")
   subject_info_template Json     @default("[]")
+  plot_config           Json?
   animals               Animal[]
 
   @@map("experiments")