fix(ci): make registry cache export non-fatal

The Forgejo registry occasionally returns 400 on large cache layer
blob uploads, failing the entire build even though the image itself
pushed successfully. Adding ignore-error=true to cache-to so cache
failures don't block deployments.

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

.forgejo/workflows/ci.yml

@@ -179,4 +179,4 @@ jobs:
           # pruned. `mode=max` exports all intermediate layers, not
           # just the final image, which is what gives the ~80% speedup.
           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=max,ignore-error=true