fix: correct Docker image tags to avoid duplicate tag names

Fixed invalid Docker tag format errors:
- Removed :preprod suffix from base image names in metadata action
- docker/metadata-action already adds branch-based tags automatically
- Unified registry to rg.fr-par.scw.cloud/weworkstudio for both backend and frontend
- This fixes "invalid reference format" error

Before: backend:preprod:preprod (invalid - duplicate tags)
After: backend:preprod (valid)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.github/workflows/ci.yml

@@ -69,7 +69,7 @@ jobs:
         id: meta
         uses: docker/metadata-action@v5
         with:
-          images: ${{ env.REGISTRY }}/backend:preprod
+          images: ${{ env.REGISTRY }}/backend
           tags: |
             type=ref,event=branch
             type=ref,event=pr
@@ -133,7 +133,7 @@ jobs:
       - name: Login to Scaleway Registry
         uses: docker/login-action@v3
         with:
-          registry: rg.fr-par.scw.cloud/xpeditis
+          registry: rg.fr-par.scw.cloud/weworkstudio
           username: nologin
           password: ${{ secrets.REGISTRY_TOKEN }}
 
@@ -141,7 +141,7 @@ jobs:
         id: meta
         uses: docker/metadata-action@v5
         with:
-          images: ${{ env.REGISTRY }}/frontend:preprod
+          images: ${{ env.REGISTRY }}/frontend
           tags: |
             type=ref,event=branch
             type=ref,event=pr