fix: set STATIC_DIR in production docker-compose for frontend serving

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -8,6 +8,7 @@ services:
     env_file: .env
     environment:
       - DATABASE_URL=postgres://ai_synth:${POSTGRES_PASSWORD}@db:5432/ai_synth
+      - STATIC_DIR=./static
     depends_on:
       db:
         condition: service_healthy