docs: add architecture guide, security model, and runbooks

[portableDD]

📋 Summary

Add the missing backend documentation required by issue #24.

Type of Change

• Documentation Update

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🎯 Requirements Checklist

Implementation

• Feature requirements have been fully implemented
• Code follows project architecture and conventions

Security

• No secrets, credentials, or environment-specific values are included

Testing

• Existing tests pass successfully

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📝 Detailed Changes

Added

• ARCHITECTURE.md — module map, request lifecycle, cross-module contracts, data flows
• RUNBOOK.md — env vars, migrations, observability stack, scheduled jobs, cache, incident checks

Updated

• README.md — accurate platform overview, links to all canonical docs
• CONTRIBUTING.md — branching strategy, contributor checklist for multi-module changes
• SECURITY_CHECKLIST.md — fills in template placeholders, covers auth/authz/CORS/uploads/logging/JWTs/rate-limiting
• SECURITY_AUDIT_REPORT.md — resolved vs open findings, removes stale placeholders
• src/wallet/README.md — links canonical docs instead of duplicating guidance
• src/common/gateways/README.md — expanded with full event table and service usage
• src/analytics-system/README.md — links config and architecture docs
• scripts/README.md — links runbook for env setup

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🧪 Testing Instructions

Steps to Verify

1. All linked documents render correctly on GitHub.
2. npm run test — same 20 pre-existing failing suites, no regressions.
3. npm run lint — same 1129 pre-existing problems, none introduced.

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🔗 Related Issues

Closes #24

[MaryammAli]

LGTM
thanks for ur contribution