ci: migrate from ghcr.io to Gitea built-in registry

- Update REGISTRY from ghcr.io to git.farh.net
- Replace Docker Hub + GHCR login with single Gitea registry login
- Use GITEA_USERNAME and GITEA_TOKEN secrets for Gitea auth

CAR-995

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ permissions:
   packages: write
 
 env:
-  REGISTRY: ghcr.io
+  REGISTRY: git.farh.net
   IMAGE_NAME: cartsnitch/api
 
 jobs:
@@ -123,19 +123,13 @@ jobs:
           echo "version=$VERSION" >> "$GITHUB_OUTPUT"
           echo "CalVer tag: $VERSION"
 
-      - name: Log in to Docker Hub
-        uses: docker/login-action@v3
-        with:
-          username: ${{ secrets.DOCKERHUB_USERNAME }}
-          password: ${{ secrets.DOCKERHUB_TOKEN }}
-
-      - name: Log in to GHCR
+      - name: Log in to Gitea Registry
         if: github.event_name == 'push' && github.ref == 'refs/heads/main'
         uses: docker/login-action@v3
         with:
           registry: ${{ env.REGISTRY }}
-          username: ${{ github.actor }}
-          password: ${{ secrets.GITHUB_TOKEN }}
+          username: ${{ secrets.GITEA_USERNAME }}
+          password: ${{ secrets.GITEA_TOKEN }}
 
       - name: Extract metadata
         id: meta