Fix deployment: remove build context from docker-compose.yml

Production servers pull the pre-built image from ghcr.io — having
build: . caused docker compose up to fail when no Dockerfile was present.

Moved build: . to docker-compose.override.yml (gitignored) which Docker
Compose automatically merges locally for dev builds. Production deployments
only need docker-compose.yml and .env.

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

.gitignore

@@ -26,6 +26,9 @@ yarn-error.log*
 .env
 .env*.local
 
+# local Docker override (build context for dev, not needed in production)
+docker-compose.override.yml
+
 # vercel
 .vercel