Feat/middleware idempotency deduplication 326

[mijinummi]

🚀 #326 Request Deduplication Middleware for Idempotency

📌 Overview

This PR introduces a request deduplication middleware to enforce idempotency across critical operations. It prevents duplicate submissions, ensures safe retries, and guarantees consistent results for repeated requests.

This is particularly important for:

• Puzzle submissions
• Point claiming
• Friend requests
• Future payment operations

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🎯 Key Features

🔑 Idempotency Key Support

• Accepts client-provided X-Idempotency-Key header (UUID recommended)
• Automatically generates keys when not provided:

Closes #326