feat(payouts): enforce configurable minimum payout via MIN_STELLAR_PAYOUT

[Petah1]

Closes #368

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Summary

• Centralized config — ConfigService now exposes minStellarPayout, loaded from MIN_STELLAR_PAYOUT (default 5). A constructor validates the value at application startup: if the env var is missing, zero, negative, or non-numeric the process fails immediately with a descriptive error rather than silently accepting a broken threshold.

• Single enforcement point — PayoutsService.assertMinimumPayout(amount) is the only place the check lives. It throws BadRequestException with a dynamic message that reflects the configured value:

  { "message": "Minimum payout amount is 5 USD equivalent." }

• All payout entry points protected — the guard is called before any balance query, Stellar account load, transaction build, signing, or on-chain submission in:

  Method	Guard position
  requestPayout	after availableBalance computed, before fee calc
  requestPayoutWithDetails	after conflict check, before all DB reads
  initiateStellarPayout	after amount-match check, before balance/TX
  processPayout	after status/wallet checks, before platform secret/TX
  batchProcessPayouts	per-payout, after status/wallet checks, before TX

• Backward compatible — existing behaviour is unchanged; operators can lower or raise the threshold via the env var with no code changes.

• Removed the previous ad-hoc parseFloat(process.env.MIN_PAYOUT_AMOUNT ?? '10') in requestPayoutWithDetails — all paths now use the same threshold from config.

• Fixed pre-existing duplicate import { FeeService } in payouts.service.ts.

Test plan

• ConfigService startup throws when MIN_STELLAR_PAYOUT is "", "0", "-5", "abc"
• ConfigService uses default 5 when env var is absent
• requestPayoutWithDetails rejects amount below threshold; error message contains configured value
• requestPayoutWithDetails proceeds when amount equals or exceeds threshold
• requestPayoutWithDetails does not query DB or create payout when validation fails
• requestPayout rejects when available balance is below threshold; payout not created
• requestPayout proceeds when balance exactly equals threshold
• initiateStellarPayout rejects below-threshold amount; no balance query or TX built
• initiateStellarPayout proceeds when amount equals threshold
• processPayout rejects below-threshold payout amount; no payout.update called
• Dynamic threshold test: error message reflects a changed minStellarPayout value at runtime

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