chore: clarify background proof failure diagnostics #1629

The post-Accepted background completion boundary in AppViewModel.completeOnchainPaymentProofInBackground safely retains durable recovery when a temporary error occurs, but its warning does not distinguish a failed proof completion from a later failed subscription refresh. The latter may happen after the proof is already complete, so “Deferred … proof completion” can misdescribe the stage.

Add a concise phase-aware diagnostic and safe error classification using existing logging patterns. Do not log raw payment payloads, addresses, credentials or arbitrary exception text. Preserve cancellation propagation, durable reconciliation and the absence of financial retries. Cover failure before completion versus failure only during the subsequent subscription refresh.

Deferred LOW review finding under the repository's Review Findings policy: #1591 (comment). The crash/recovery fix itself remains in #1591.