Optional second destination so each heartbeat and event is posted to a
mirror server (the office NAS) alongside the primary — dual-write to
de-risk the migration. Default off (blank mirror URLs); existing installs
unchanged.
- event_forwarder: mirror_reachable() fast-fail probe + mirror_forward_pass()
(reliable event mirror with its OWN state file, total exception isolation,
never raises into the primary path).
- series3_watcher: MIRROR_API_URL/MIRROR_SFM_URL/MIRROR_SFM_STATE_FILE config;
best-effort heartbeat mirror; isolated event-mirror pass after the primary.
- settings dialog + config-template: new 'Mirror' tab / keys.
- 8 new tests incl. the isolation invariant. 44 passing.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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