Track gateway fees and activity for Explorer dashboards #187

Context:
To support a first-class Gateway section in the Explorer, we need reliable tracking of gateway fees and activity over time. This includes daily snapshots and rolling 30/60/90-day aggregates, similar to existing orchestrator metrics, as well as access to the active gateway set each round.

Goal:
Add gateway fee tracking with daily snapshots and rolling-window aggregates, store the active gateway set on Protocol, and expose this data via the subgraph so it can power Gateway overview and profile pages in the Explorer.