fix(payment): forward-port quote-skew tolerance 60s -> 300s (re-applies #83)

[grumbach]

Summary

Forward-port of #83 (merged 2026-04-28 into the now-superseded rc-2026.4.3 branch). The fix never made it onto rc-2026.4.4 or main, so the released 0.11.x binary still runs the old QUOTE_CLOCK_SKEW_TOLERANCE_SECS = 60 check.

This is the dominant prod failure mode in Chris's 2026-05-05/06/07 PROD-UL-01 / STG-01 failure-analysis runs: storers reject quotes whose timestamp is in the future beyond the 60s tolerance, which fires regularly under realistic operator-clock drift.

Cherry-picks Mick's commit verbatim (069f6bc):

• validate_quote_timestamps had an unreachable else arm: when now.duration_since(quote.timestamp) returns Err, quote.timestamp.duration_since(now) necessarily returns Ok, so the "invalid timestamp" path could never fire. Replaced with SystemTimeError::duration().
• Replaced Duration::as_secs() > CONST with Duration > Duration::from_secs(CONST). The old form floored fractional seconds, silently widening both windows by up to ~1 s past the documented limits.
• Raised the forward-skew tolerance from 60 s to 300 s and renamed QUOTE_CLOCK_SKEW_TOLERANCE_SECS → QUOTE_FUTURE_SKEW_TOLERANCE_SECS. 60 s was too tight for unmanaged P2P nodes with imperfect clock sync; 300 s gives realistic drift headroom while remaining negligible against the 24 h max age. Past-side skew is intentionally absorbed entirely by QUOTE_MAX_AGE_SECS — there is no separate past tolerance.

No public API change. Behaviour change is limited to which forward-dated quotes are accepted.

Test plan

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --lib payment::verifier — 44/44 pass

Will validate against prod failure rate once the next testnet run lands.

[Copilot]

Pull request overview

This pull request forward-ports the payment quote freshness fix from #83 to ensure production nodes accept realistically forward-dated quotes under common wall-clock drift, while still rejecting stale and excessively future quotes.

Changes:

• Increase tolerated forward quote timestamp skew from 60s to 300s and rename the constant to reflect “future skew” semantics.
• Simplify and correct future-timestamp handling in validate_quote_timestamps by using SystemTimeError::duration() and removing the previously unreachable “invalid timestamp” path.
• Make timestamp comparisons precise by comparing Duration values directly (avoiding as_secs() flooring in the decision logic) and update the affected tests accordingly.

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