feat: implement AI dispute mediation, webhook DLQ, GSN gas sponsorship, and collaborative config editing

[akintewe]

Summary

This PR implements four high-priority features across the AgenticPay platform:

Issue #452 — AI-Powered Payment Dispute Mediation System

• backend/src/services/disputes/ai-mediator.ts: NLP signal extraction pipeline (fraud, non-delivery, quality, duplicate patterns), confidence-scored resolution classification, historical pattern boost, auto-resolution at >95% confidence, human escalation with AI-generated summary
• backend/src/services/disputes/resolution-engine.ts: Full dispute lifecycle, SLA enforcement (72h deadline), mediator decision recording for continuous model improvement, analytics dashboard (resolution breakdown, SLA breach count, average resolution hours)
• backend/src/routes/disputes-ai.ts: REST endpoints — POST /:id/mediate, POST /:id/human-resolve, GET /escalation-queue, GET /analytics, GET /:id/mediation-log

Issue #453 — Webhook Event Replay and Dead-Letter Queue Management

• backend/src/services/webhooks/delivery-manager.ts: Configurable retry policy per endpoint (max retries, backoff multiplier, timeout, window days), delivery logs with status/latency/payload preview, success-rate health dashboard with <99% alert, CSV export
• backend/src/services/webhooks/dead-letter-queue.ts: DLQ with permanent failure categorization (8 categories), single/batch/all replay, purge operations, sync from delivery logs, DLQ stats
• backend/src/routes/webhooks-dlq.ts: REST endpoints — health, delivery-logs, CSV export, DLQ CRUD, replay (single/batch/all), endpoint retry policy management

Issue #454 — Gas Station Network for EVM Gas Sponsorship

• contracts/evm/gsn/GSNForwarder.sol: EIP-2771/EIP-712 trusted forwarder with authorized relayer management, on-chain per-sender nonce tracking, replay protection
• contracts/evm/gsn/SponsorshipPolicy.sol: On-chain merchant budget deposits, per-wallet daily rate limiting, relay-server billing integration, budget exhaustion events
• backend/src/services/gsn/relay-server.ts: Meta-tx validation (signature format, deadline, replay protection), EIP-1559 fee estimation with configurable multiplier, nonce store, fallback-to-user-gas when budget exhausted
• backend/src/services/gsn/budget-manager.ts: Per-merchant budget CRUD, top-up, rate-limit enforcement (rolling 24h window), gas spend recording, billing summaries
• backend/src/routes/gsn.ts: REST endpoints — relay, estimate, budget management, billing summary, sponsored tx list

Issue #455 — Real-Time Collaborative Project Config Editing

• backend/src/websocket/collaboration.ts: Socket.IO namespace handler with OT-based operation sync (<200ms), presence tracking with cursor positions, critical field locking (apiKey, webhookUrl, signingSecret), offline op queue/replay on reconnect, edit history with per-user attribution and rollback
• backend/src/routes/collaboration.ts: REST endpoints — edit history, active participants, locked fields
• frontend/components/collaboration/useCollaboration.ts: React hook — Socket.IO client, operation send with 300ms debounce, offline queue flush on reconnect, lock acquire/release Promise API
• frontend/components/collaboration/PresenceIndicators.tsx: Avatar stack with cursor field tooltip, active dot indicator, overflow counter
• frontend/components/collaboration/CollaborativeField.tsx: Input with remote cursor indicator, lock state visual styles, critical field lock acquire/release on focus/blur
• frontend/components/collaboration/EditHistoryPanel.tsx: Slide-in panel with per-user attributed history, version display, rollback with confirmation dialog
• frontend/components/collaboration/CollaborativeProjectConfig.tsx: Full project config form wired to collaboration hook — general, redirect, and critical security fields

Test plan

• Trigger AI mediation on a dispute and verify auto-resolve at high confidence vs. escalation queue at low confidence
• Submit events to a webhook endpoint, verify DLQ population on failures, replay single/batch entries
• Create a merchant GSN budget, submit meta-transaction, verify budget deduction and fallback when exhausted
• Open project config as two users simultaneously, verify real-time sync, presence indicators, lock behavior on critical fields, and history rollback

closes #452 closes #453 closes #454 closes #455

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