feat: implement background judge worker (#6)

[anoncon]

Implements the background judge worker that polls open disputes every 30 seconds, calls the AI judge service, persists verdicts, resolves disputes on-chain via Soroban, and resets failed disputes back to open.

• Implement run_judge_worker with 30s polling loop
• Implement JudgeService::judge() via configurable HTTP endpoint (JUDGE_API_URL)
• Spawn worker from main.rs alongside HTTP server
• Integration test with mocked judge API (wiremock)

Closes #6

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[anoncon]

@soomtochukwu please review my PR