[Penielka]

Summary

• Adds a centralized PuzzleSolutionCheckerService that validates off-chain puzzle answers against the server-side solution before on-chain submission.
• Normalizes answers deterministically (Unicode NFKC, trim, case-insensitive) and compares SHA-256 digests with timingSafeEqual.
• Rate limits checks per player/IP and puzzle, returning 429 when the limit is exceeded.
• Persists every attempt (including rate-limited and unknown-puzzle attempts) with a hashed answer, outcome, IP, user agent, and duration for abuse detection. Raw answers are never stored.
• Exposes POST /api/puzzle-solution/check.
• Repairs the corrupted backend/src/main.ts bootstrap that prevented the Nest app (and new module) from compiling.

Tests

cd backend
npx jest src/puzzle-solution-checker --runInBand
npx eslint "src/puzzle-solution-checker/**/*.ts" "src/main.ts" "src/app.module.ts"

All 8 new tests pass. tsc -p tsconfig.build.json still reports pre-existing errors in unrelated modules.

Closes #713
closes #708
closes #760
closes #719

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[dzekojohn4]

LGTM