COINBASE_MATURITY=30 + L2 /submit stops mutating SMT

Bitcoin uses COINBASE_MATURITY=100 (you can't spend a coinbase output until 100 blocks deep). With our 1-minute target block time, that's 100 minutes of waiting before mining rewards become spendable. Cut to 30 for a smoother dev/UX experience. Still safe — the reorg history on Vanta is dominated by 1-deep reorgs. Bigger fix in the same commit: the L2 /submit endpoint was mutating the sparse…