Feature/early exit validation ownership

[MerlinTheWhiz]

Closes #434

Changes made:

• src/app/api/commitments/[id]/early-exit/route.ts: added request-body validation, auth/ownership enforcement, and standard API error handling for contract failures.
• src/lib/schemas/apiContracts.ts: added EarlyExitRequestBodySchema for reason and callerAddress.
• src/lib/backend/services/contracts.ts: wired early-exit failures through normalizeContractError.
• tests/api/early-exit.test.ts: added/updated coverage for valid request, invalid body, unauthorized, forbidden ownership, rate limit, and normalized contract error cases.
• docs/backend-api-reference.md: documented early-exit auth, body schema, error cases, and corrected example payload.

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