feat: add idempotency and duplicate-submission protection

[m-hajjo]

Problem

Players could submit the same answer multiple times (double-clicks, network retries, browser refreshes, or malicious requests), causing duplicate XP awards, double session advancement, and duplicate reward eligibility.

Solution

Introduced a Redis-backed IdempotencyService using SETNX-based lock + result caching. Every write path now wraps its core logic in idempotency.execute(key, fn), which guarantees the function runs at most once per key within a configurable TTL (default 24h).

Protocol:

1. Check for a cached result → if found, return it (duplicate: true)
2. Attempt to acquire a Redis lock (SETNX, 60s TTL) → if acquired, run fn, store the result, release the lock
3. If the lock is held by a concurrent request → poll until the result appears or the lock expires, then force-acquire

Closes #626

[vercel]

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
mind-block-app-frontend	Skipped		Aug 24, 2026 10:57am

[phertyameen]

@m-hajjo can you please resolve conflict?

[vercel]

@m-hajjo is attempting to deploy a commit to the aminubabafatima8-gmailcom's projects Team on Vercel.

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