feat(nodectl): refresh past_elections cache periodically

[Keshoid]

Summary

• Adds a TTL-based refresh for the elections runner's past_elections and pool-address caches (currently invalidated only on election_id change, i.e. once per ~9h round).
• Defends against the SMA-98 incident where a stale snapshot from a lagging RPC endpoint persisted for the whole round and inflated frozen_stake to 2× the real value.
• New config field elections.cache_refresh_secs (default 300, set 0 to disable). Hot-reloaded via the 10s config-file watch.
• Logs past_elections cache refreshed (reason=election_id|ttl, ...) on each refresh so the trigger is visible in the daemon log.
• Clock trait introduced in common (production SystemClock, deterministic MockClock) so the TTL logic is unit-testable without real-time sleeps.
• Singlehost e2e (run_singlehost_nodectl.py) sets cache_refresh_secs=60 after config generate so the periodic refresh is exercised in the e2e log.

Closes SMA-98.

[linear]

SMA-98

[Copilot]

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