add dispute resolution integration tests for #272

[Franklivania]

Description

Adds integration test coverage and backend support for the full dispute → resolution flow (issue #272).

Contributors can dispute a submitted bounty; a configured arbiter can resolve it by releasing funds to the contributor (release: true) or refunding the maintainer (release: false). Supertest tests verify status transitions, audit log entries for both paths, and that a non-arbiter signer receives 401.

Changes:

• POST /api/bounties/:id/resolve-dispute route with resolveDisputeSchema and OpenAPI registration
• resolveDisputeBounty() in the bounty store (disputed → released / refunded + audit logs)
• Parameterized Stellar signature auth for maintainer, contributor (dispute), and arbiter (resolve) roles
• Extended backend/test/api.dispute.test.ts with resolve and wrong-arbiter auth cases
• Restored backend/src/middleware/auth.ts and backend/src/metrics.ts (were empty/incomplete on main, blocking app import and tests)

Closes #272

Type of change

• Bug fix
• New feature
• Breaking change

Security Checklist

Please review the SECURITY_CHECKLIST.md and check off any items that apply. Reviewers must sign off on these items before merge.

• Input validation changed
• Auth modified
• New external fetch
• Dependency added
• Secret handling

Test plan

• cd backend && npm test -- test/api.dispute.test.ts — 9/9 pass (dispute, resolve release/refund, audit logs, wrong-arbiter 401)
• cd backend && npm test -- test/authMiddleware.test.ts — maintainer signature auth unchanged
• cd backend && npm test -- test/api.test.ts — lifecycle routes pass (open-issues timeout is pre-existing)

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