Bounty Builder Lifecycle

Builder/participant lifecycle for bounties in v1 boundless: discover -> apply/join/claim -> submit -> withdraw (application/submission) -> reward receipt. Mirrors the hackathon participant patterns. Reward stays push-model (select_winners pays winners on-chain; 'reward withdrawal' = the receipt/earnings view). Full-stack: fills the backend participant gaps + builds the v1 builder UI. Covers all six modes (entryType x claimType).