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>

.github/workflows/ci.yml

@@ -118,7 +118,8 @@ jobs:
       - name: Build Backend Docker image
         uses: docker/build-push-action@v5
         with:
-          context: ./backend
+          context: .
+          file: backend/Dockerfile
           push: false
           tags: trendyol-backend:test
           cache-from: type=gha
@@ -164,7 +165,8 @@ jobs:
 
             backend:
               build:
-                context: ./backend
+                context: .
+                dockerfile: backend/Dockerfile
               environment:
                 PYTHONUNBUFFERED: 1
                 DATABASE_URL: postgresql://postgres:testpassword@postgres:5432/trendyol_db