fix(ci): make docker push retry more aggressive against Gitea registry

The Gitea registry sometimes returns 'net/http: timeout awaiting
response headers' on blob uploads. The existing retry loop (3x15s)
isn't enough.

- Bump retries 3 -> 5
- Backoff 15s -> 30s
- --max-concurrent-uploads=1 to avoid hammering a small registry

Total worst-case wait: ~4 minutes (4 retries * 30s + 5 pushes ~ 30s).

.gitea/workflows/ci.yml

@@ -90,14 +90,14 @@ jobs:
           push_with_retry() {
             local ref="$1"
             local attempt=1
-            local max_attempts=3
+            local max_attempts=5
             while [ $attempt -le $max_attempts ]; do
               echo "=== Push (attempt $attempt/$max_attempts): $ref ==="
-              if docker push "$ref"; then
+              if docker push --max-concurrent-uploads=1 "$ref"; then
                 return 0
               fi
-              echo "::warning::Push of $ref failed, retrying in 15s..."
-              sleep 15
+              echo "::warning::Push of $ref failed, retrying in 30s..."
+              sleep 30
               attempt=$((attempt + 1))
             done
             echo "::error::Push of $ref failed after $max_attempts attempts"