Clarify fee volume fields #188

As mentioned by @jmulq in #168 (review), the current volume* fields (e.g. Protocol.totalVolumeETH, BroadcasterDay.volumeETH) are used to track fee payouts (such as winning ticket redemptions), but the term “volume” is ambiguous for downstream consumers and may be misinterpreted.

A possible improvement would be to introduce more explicit fields (e.g. totalFeesPaidETH / totalFeesPaidUSD) and populate them alongside the existing volume* fields. The existing fields should be kept for backward compatibility, with clear documentation of their meaning and a plan to deprecate them over time.