StashClient: cancellable requests, longer sync timeout, defensive parsing

Uses the new Call.await() instead of a thread-blocking execute(). The
sync=true save endpoint can take up to ~30s (server-side extract +
summary), longer than the shared client's stock 10s read timeout --
overridden per-call so feed fetches don't inherit a 35s timeout they
don't need. A 409 (duplicate) is authoritative from the status code
alone; a body that doesn't match DuplicateErrorResponse's exact shape
no longer downgrades a real "already saved" into a misleading
"save failed".

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