Andrew7234/staking rewards api

[Andrew7234]

Adds an endpoint to fetch the staking rewards for a given account in a given calendar year.

Todo:
[] performance testing, especially on a large db with a full year's worth of data

Alternatives considered:

• injecting synthetic events for each staking reward (delegator x delegatee x epoch). I didn't like this because it bloats the events table. At the start of 2025 there were ~62300 unique delegations, which over the course of a year would lead to 62300 * 365 * 24 = ~545 million synthetic events per year, not including prior history or new delegations that are created. Open to revisiting this approach or exploring alternatives based on the results of the performance tests though.