Add a security best practices guide for platform integrators

[Mystery-CLI]

🔧 Title: Add a security best practices guide for platform integrators

📘 Description
There is no security guide for developers integrating the FuTuRe platform into their own applications. Without documented guidance, integrators may handle API keys, private keys, and webhook signatures insecurely, putting user funds at risk.

✅ Acceptance Criteria

• Create docs/guides/security.md covering: API key storage and rotation, webhook signature verification, private key management (never log, never transmit, hardware wallet recommendations), CSP configuration, known attack vectors (replay attacks, front-running, sequence number manipulation)
• Add a link to the guide from README.md under a Guides section
• Add a link from the Swagger UI description
• Review must be completed by a security-aware contributor before merging

🔧 Context: README.md; backend/CONFIGURATION.md; backend/src/config/swagger.js.

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Total: 155 issues across 15 categories.