fix: bundle categories into Docker image for Coolify deployment

Coolify remaps volume mounts to its own managed directories, so
./categories was mapped to an empty dir instead of the repo data.

Changes:
- Backend build context changed to repo root (.) so categories/ is accessible
- Dockerfile copies categories into /data/initial-categories/
- startup.sh seeds /data/categories from bundled data if empty
- Removed categories volume mount (reports still persisted via volume)
- Added root .dockerignore (categories NOT excluded)
- Updated CI workflow to match new build context

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

docker-compose.yml

@@ -30,8 +30,8 @@ services:
   # ============================================
   backend:
     build:
-      context: ./backend
-      dockerfile: Dockerfile
+      context: .
+      dockerfile: backend/Dockerfile
     restart: unless-stopped
 
     ports:
@@ -52,8 +52,7 @@ services:
       FRONTEND_URL: http://trendyol.194.187.253.230.sslip.io
 
     volumes:
-      # Persistent data volumes
-      - ./categories:/data/categories
+      # Persistent data volumes (reports persist across deploys)
       - ./reports:/data/reports
 
     depends_on: