feat(storage): sccache S3 build cache with scoped SeaweedFS identity

Add sunbeam-sccache bucket and a dedicated sccache S3 identity scoped
to Read/Write/List/Tagging on that bucket only. Bump volume server
max from 50 to 100 (was full, blocking all new writes).

base/lasuite/seaweedfs-buckets.yaml

@@ -30,7 +30,8 @@ spec:
                 sunbeam-git-lfs \
                 sunbeam-game-assets \
                 sunbeam-ml-models \
-                sunbeam-stalwart; do
+                sunbeam-stalwart \
+                sunbeam-sccache; do
                 mc mb --ignore-existing "weed/$bucket"
                 echo "Ensured bucket: $bucket"
               done