fix(ci): migrate Docker registry from GHCR to Gitea

- Change REGISTRY from ghcr.io to git.farh.net
- Replace GHCR login with Gitea Container Registry login using REGISTRY_TOKEN
- Move workflow from .github/workflows to .gitea/workflows

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

.gitea/workflows/ci.yml

@@ -15,7 +15,7 @@ permissions:
   packages: write
 
 env:
-  REGISTRY: ghcr.io
+  REGISTRY: git.farh.net
   IMAGE_NAME: cartsnitch/receiptwitness
 
 jobs:
@@ -69,12 +69,12 @@ jobs:
           echo "version=$VERSION" >> "$GITHUB_OUTPUT"
           echo "CalVer tag: $VERSION"
 
-      - name: Log in to GHCR
+      - name: Log in to Gitea Container Registry
         uses: docker/login-action@v3
         with:
           registry: ${{ env.REGISTRY }}
           username: ${{ github.actor }}
-          password: ${{ secrets.GITHUB_TOKEN }}
+          password: ${{ secrets.REGISTRY_TOKEN }}
 
       - name: Extract metadata
         id: meta