feat(#549/#550/#552/#555): fee-bump, SEP-41 allowances, path payment,…

[Jayy4rl]

… operator monitoring

Closes #555 Fee-bump / sponsored transactions:

• POST /api/v1/fee-bump wraps a signed inner tx XDR with a sponsor fee-bump
• Op allowlist (invokeHostFunction, payment, claimClaimableBalance, changeTrust, createAccount)
• Per-wallet daily quota (default 10, configurable via FEE_BUMP_DAILY_QUOTA)
• Circuit breaker: rejects when sponsor XLM < MIN_SPONSOR_BALANCE
• GET /api/v1/fee-bump/quota/:wallet to inspect remaining quota
• Migration 025_fee_bump_quota.js tracks daily usage per wallet
• Frontend helpers: isEligibleForSponsorship(), wrapWithFeeBump()

Closes #552 Operator balance monitoring:

• operatorBalanceService checks XLM balance of operator/sponsor accounts via Horizon
• Emits structured log.warn + metrics.operatorLowBalance when below threshold
• Optional auto-topup (guarded by AUTO_TOPUP_ENABLED env flag)
• operatorBalanceJob polls every 5 min (OPERATOR_BALANCE_CHECK_INTERVAL_MS)
• Migration 026_operator_balance_log.js stores per-check audit history

Closes #550 SEP-41 allowance / approve / transfer_from flows:

• 8 new Rust tests in contracts/rewards/src/test.rs covering: approve + allowance read, transfer_from consumes allowance, over-spend rejection, expiration rejection, re-approve resets amount+expiry, zero-amount approve clears allowance, burn_from, token mode gate
• Frontend helpers: submitApproveTransaction, fetchAllowance, submitTransferFromTransaction, submitBurnFromTransaction
• docs/CONTRACTS_API.md: new SEP-41 section with edge-case table, mutual exclusivity note, and TypeScript usage examples

Closes #549 Path payment for multi-asset claims:

• GET /api/v1/payment-paths proxies Horizon /paths/strict-receive with trustline pre-check and structured error for missing trustline
• POST /api/v1/payment-paths/claim validates slippage (0-500bps, default 100) and returns adjusted maxSendAmount for wallet to sign
• Frontend helpers: fetchPaymentPaths(), submitPathPaymentClaimTransaction()
• Integration tests in backend/src/integration/feeBump.test.js

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