Fix plugin payout currency mismatch for non-USD developers

[simonhamp]

Problem

Every plugin payout for a developer whose payout_currency is not USD was failing in production with:

The currency of source_transaction's balance transaction (usd) must be the same as the transfer currency (eur)

When a transfer is linked to a specific charge via source_transaction, Stripe requires the transfer's currency to match the source charge's currency. We were instead setting the transfer currency to the developer's payout_currency (e.g. EUR), while the source charge was in the customer's payment currency (USD) — so the two never matched and the payout was marked failed.

This affected every non-USD developer, not just a one-off.

Fix

StripeConnectService::processTransfer() now uses the source charge's currency as the transfer currency. Stripe handles FX to the connected account's preferred payout currency on the destination side.

• Renamed getChargeIdFromPayout() → getChargeDetailsFromPayout(), returning both the charge id and currency from the PaymentIntent (no extra Stripe round-trip).
• Falls back to the developer's payout_currency when no payment intent is linked to the license.
• Added currency to the success log for easier future debugging.

Tests

Added tests/Feature/Services/StripeConnectServiceTest.php:

• EUR developer + USD charge → transfer created in usd with source_transaction set; payout marked transferred.
• License without a payment intent → falls back to the developer's payout_currency and omits source_transaction.

Follow-ups (not in this PR)

• Existing failed payout WIP: Homepage #2 is stuck at status = failed; it needs payouts:retry-failed --payout-id=2 (or a manual reset to pending) once this deploys.
• The ProcessPayoutTransfer job's $tries = 3 is inert because processTransfer() swallows its own exceptions — the queue never sees a failure to retry.
• PluginPayout has no failure_reason/failed_at/attempts columns, so failures are only visible in logs.

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