fix: downgrade buildx cache export from mode=max to mode=min

mode=max exports all intermediate layer blobs which exceeded the Forgejo
registry's blob upload size limit (400 Bad Request). mode=min only exports
the final image layers, keeping cache entries small enough for the registry.
The actual image build and push was succeeding; only the cache write failed.

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

.forgejo/workflows/ci.yml

@@ -143,4 +143,4 @@ jobs:
           tags: ${{ steps.tags.outputs.value }}
           build-args: BUILD_VERSION=${{ steps.tags.outputs.build_version }}
           cache-from: type=registry,ref=${{ env.IMAGE }}:cache
-          cache-to: type=registry,ref=${{ env.IMAGE }}:cache,mode=max
+          cache-to: type=registry,ref=${{ env.IMAGE }}:cache,mode=min