MB.1 Member offered bounties

Members offer and fund bounties for Library items (today only staff post bounties, and staff pay by hand or by a Stripe Transfer from our own balance, test mode). A member-funded bounty needs escrow: (1) the poster pays first through Stripe Checkout and the bounty goes live only once the money is received; the funds sit in our Stripe balance tagged to the bounty. (2) The winner needs a Stripe Connect Express account (the maker payout setup); awarding a claim prompts them to set one up. (3) On accept, a Transfer (separate charges and transfers, the winner is unknown at payment time) sends the reward minus our fee, after a short hold against chargebacks. (4) Refund to the poster if the deadline passes with no accepted claim, or if they withdraw before any award. (5) The item still goes into the public Library under CC0 or CC BY; the poster gets no exclusive rights. Open owner decisions: who accepts (poster, staff only for disputes: recommended), our fee, who absorbs Stripe's fee on a refund. Reuses Makers, AddonMoney (test-mode guard), LibraryBounties::payWithStripe and the bounty tax-form threshold. Builds on LIB-BOUNTY (0.9).