docs: add mainnet launch readiness checklist

docs/MAINNET_CHECKLIST.md

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+# Mainnet Launch Readiness Checklist
+
+This checklist must be completed and reviewed before Trivela launches on mainnet.
+Each item should be checked off by a responsible contributor and linked to evidence
+(PR, audit report, deployment tx, etc.) before the launch is approved.
+
+---
+
+## Smart Contracts
+
+- [ ] `upgrade()` entrypoint implemented and tested — [#278]
+- [ ] TTL values set for mainnet timing — [#279]
+- [ ] Participant storage migrated to persistent storage — [#280]
+- [ ] 2-step admin transfer implemented — [#281]
+- [ ] All fuzz targets passing — [#282]
+- [ ] Formal security audit completed (external auditor, report linked)
+- [ ] Contracts deployed and verified on mainnet
+
+---
+
+## Backend
+
+- [ ] Event indexer running against mainnet RPC — [#283]
+- [ ] PostgreSQL or hardened SQLite configured — [#284]
+- [ ] Database migration system in place — [#286]
+- [ ] Redis caching configured for multi-instance — [#288]
+- [ ] Rate limits tuned for expected traffic
+- [ ] All secrets rotated from testnet values
+- [ ] Monitoring and alerting configured — [#290]
+
+---
+
+## Frontend
+
+- [ ] Mainnet contract IDs set in `VITE_*` env variables
+- [ ] All Freighter/wallet errors surface clear user messages
+- [ ] No hardcoded testnet references remain
+- [ ] Performance targets met: LCP < 2.5s, CLS < 0.1
+
+---
+
+## Infrastructure
+
+- [ ] Mainnet deploy pipeline with approval gates configured — [#289]
+- [ ] Backup strategy for campaign DB documented and tested
+- [ ] Incident response runbook documented
+- [ ] `SECURITY.md` published — [#292]
+
+---
+
+## References
+
+- [DEPLOYMENT.md](./DEPLOYMENT.md)
+- Issues: #278, #279, #280, #281, #282, #283, #284, #286, #288, #289, #290, #292