revert: remove out/ named volume (causes EBUSY with Next.js cleanDistDir)

Next.js tries to rmdir the entire out/ directory before rebuilding
(cleanDistDir: true). Docker mount points can't be removed from inside
containers. The root-owned files were fixed via one-time chown; USER
node in the Dockerfile prevents recurrence.

Reverts the out_cache addition from c571896.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Dev User

docker-compose.yml

@@ -12,7 +12,6 @@ services:
       - .:/app
       - node_modules:/app/node_modules
       - next_cache:/app/.next
-      - out_cache:/app/out
     env_file:
       - .env
     environment:
@@ -58,4 +57,3 @@ volumes:
   postgres_data:
   node_modules:
   next_cache:
-  out_cache: