distribute rewards per epoch to bound single-prediction capture

[trizin]

Reward Allocation Change

Previous design: Rewards were allocated proportionally to each
predictoor's total weekly profit per feed.

Problem: Because profit is just stake redistribution, an attacker could
both-side a market across unlinkable wallets to manufacture a huge
positive-profit wallet for ~zero net cost — and thereby capture the entire
feed budget.

New design: Split the budget across three stages:

1. Equally across feeds (contracts)
2. Equally across all possible weekly epochs for that feed
3. Within each epoch, proportional to positive profit in that epoch