fix: align relayer fee-bump route for issue 32

[Maximum-Prosper]

Summary

Fix the gasless onboarding flow for issue #32 by aligning the fee-bump relayer route across the backend and callers.

What changed

• Added a backward-compatible backend alias for the legacy fee-bump endpoint:
  • /relay/sign-fee-bump
  • /api/relayer/fee-bump
• Updated the rebalance executor to call the canonical relayer route.
• Kept the frontend fee-bump calls on the canonical relayer route.
• Added route coverage to prevent regressions.

Why

The relayer handler existed, but one server-side caller still used the legacy /relay/sign-fee-bump path while the backend mounted /api/relayer/fee-bump. That mismatch could break fee-bumped transactions for onboarding and rebalance flows.

Verification

• Added regression coverage for both:
  • server/src/__tests__/routeAudit.test.ts
  • server/src/__tests__/deploymentSmoke.test.ts
• Confirmed the code paths now use the same canonical fee-bump endpoint.

Notes

• The legacy alias is kept temporarily for compatibility.
• This preserves existing behavior while reducing the chance of another route mismatch.warnings

Closes #32

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