Remove build: directives — images pulled from Gitea registry only

All builds now go through the CI/CD pipeline. The compose file
only needs image: to pull pre-built images from the registry.

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

docker-compose.yaml

@@ -23,7 +23,6 @@ services:
 
   backend:
     image: git.sentinelforest.xyz/rohskiddo/umbra-backend:main-latest
-    build: ./backend
     restart: unless-stopped
     environment:
       - DATABASE_URL=postgresql+asyncpg://${POSTGRES_USER}:${POSTGRES_PASSWORD}@db:5432/${POSTGRES_DB}
@@ -54,7 +53,6 @@ services:
 
   frontend:
     image: git.sentinelforest.xyz/rohskiddo/umbra-frontend:main-latest
-    build: ./frontend
     restart: unless-stopped
     ports:
       - "80:8080"