fix: preserve lnurl msat precision

[piotr-iohk]

Summary

• preserve LNURL-pay invoice precision by creating LND invoices with value_msat instead of truncating callback amounts to sats
• preserve LNURL-withdraw range validation precision by validating callback invoices against num_msat (with sat fallback)
• add changelog entries under Unreleased documenting LNURL pay/withdraw msat precision fixes

Test plan

• Start LNURL server and generate fixed LNURL-pay with sub-sat amount (e.g. 222222/222538 msat)
• Decode created invoice and verify num_msat preserves exact callback amount
• Generate fixed LNURL-withdraw with sub-sat range and verify callback validation accepts in-range num_msat
• Verify out-of-range LNURL-withdraw callbacks are rejected with msat range message

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