fix: apply reward routing budget to payouts

[keshav0479]

What does this PR do?

Related to #1382
Follow-up to #2038 and #2364

While rebasing #2364, I looked at the reward backend path again.

#2038 added routing_budget_ppm to /api/reward/, but the reward LNPayment was not storing the budget fields. So the invoice amount could be checked with the submitted budget, while the actual LND/CLN reward payout still used the default reward fee cap.

This stores the reward budget on the LNPayment and uses the stored routing_budget_sats when paying rewards.

I also fixed a failure path where reward payout could return plain False, but withdraw_rewards() expected (paid, failure_reason). In that case the reward balance had already been cleared, so now it returns a normal invoice error and restores the original reward balance.

Tested:

• ./venv/bin/python manage.py test api.tests.test_logics --verbosity 2
• ./venv/bin/ruff check api/logics.py api/serializers.py api/lightning/lnd.py api/lightning/cln.py api/tests/test_logics.py
• git diff --check upstream/main

Checklist before merging

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