Detect configs expired from the swarm and re-store them

[mpretty-cyro]

Config messages have a 30-day TTL that is refreshed by the expire bump already piggybacked on every poll. A device offline past the TTL loses its config from the swarm and nothing noticed. This reads the bump's own answer to detect that case and re-stores from local state. Reasoning for each rule is in the code.

• Detect missing config hashes from the unchanged array the server returns for an extend-only expire. An absent key means detection is unavailable, not that every config is gone.
• Re-store on a later poll, only once local state is level with the swarm, so a stale local config can never overwrite a newer remote one.
• Chunk the recovery batch at 20 sub-requests; the server rejects the whole sequence above that, and a single config can split into ~66 parts.
• Run the obsolete-hash deletes as a separate pass after the stores, built only from restores that fully landed, so a delete can never go out over a config we did not finish storing.
• Bar a settled hash for an hour rather than for the session; a backgrounded session can outlive the TTL, which would strand the config it just restored.
• Drop the per-store semaphore and let BatchApiExecutor coalesce; the semaphore was bounding batch size by accident, not by design.