feat: best-effort mirror (dual-send) of heartbeats + events

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).
- series4_ingest: mirror_api_url/mirror_sfm_url/mirror_sfm_state_file config;
  best-effort heartbeat mirror; isolated event-mirror pass after the primary.
- settings dialog: new 'Mirror' tab.
- 9 new tests incl. the isolation invariant (down mirror = fast no-op that
  never touches primary state). 42 passing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

config.example.json

@@ -24,5 +24,9 @@
   "sfm_http_timeout": 60,
   "sfm_state_file": "",
   "sfm_max_forwards_per_pass": 500,
-  "sfm_max_event_age_days": 365
+  "sfm_max_event_age_days": 365,
+
+  "mirror_api_url": "",
+  "mirror_sfm_url": "",
+  "mirror_sfm_state_file": ""
 }