Fix ambiguous Data.ByteString.Base64 import in Omni/Health/Intervals

typecheck.sh Omni/Health/Web.hs fails before reaching Web due Omni/Health/Intervals.hs importing Data.ByteString.Base64 ambiguously (provided by packages base64 and base64-bytestring). This blocks normal verification of health dashboard changes. Resolve the import to a specific package/module and restore typecheck.