fix(opencode): stop submitting expired verified protos

[bmc08gt]

Bugsnag 6aa188d3ce44e305cadc4a0a: a withdrawal failed three times with SubmitIntentError.StaleState: exchange rate is stale. The protos in the request were signed about 23 minutes before submission, and the server rejects anything older than 15.

VerifiedProtoManager.getOrEvict already checks the timestamp against a 15-minute TTL. When it finds an expired entry it evicts it from the map and then returns it anyway. RealVerifiedFiatCalculator.resolveVerifiedState treats any non-null result as usable, so the fresh-fetch branch never ran: none of the three attempts logged "Fetching fresh". The live-mint stream re-saved the same old-stamped protos every few seconds, so each retry hit the same path.

Three changes in the manager:

• getOrEvict returns null for an expired entry, so the calculator falls through to getLiveMintData and either gets a fresh proof or fails with StaleRate before the request goes out.
• The ceiling drops from 15 minutes to 13, matching iOS clientMaxAge and leaving headroom for the request to land inside the server's window.
• saveRates and saveReserveStates skip protos that arrive already past the ceiling, the same check iOS makes in VerifiedProtoService on arrival.

kotlin.time.Clock is injected through an internal primary constructor; the no-arg @Inject constructor delegates to Clock.System, so Hilt and the two VerifiedProtoManager() factory call sites are unchanged. VerifiedProtoManagerTest drives the expiry boundary with a fake clock, and RealVerifiedFiatCalculatorTest gains a case showing a null cache hit triggers the fetch.

This does not explain why the stream kept sending protos stamped 23 minutes old. If the backend keeps doing that, the refetch will fail with StaleRate instead of succeeding, which is still the right outcome for the user's balance but leaves the backend question open.