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>

thor_settings_dialog.py

@@ -47,6 +47,11 @@ DEFAULTS = {
     "sfm_state_file":                   "",
     "sfm_max_forwards_per_pass":        500,
     "sfm_max_event_age_days":           365,
+
+    # Mirror (dual-send) — best-effort second destination, default OFF.
+    "mirror_api_url":         "",
+    "mirror_sfm_url":         "",
+    "mirror_sfm_state_file":  "",
 }
 
 
@@ -220,6 +225,14 @@ class SettingsDialog:
         self.var_sfm_max_age_days       = tk.StringVar(value=str(v.get("sfm_max_event_age_days", 365)))
         self.var_sfm_state_file         = tk.StringVar(value=str(v.get("sfm_state_file", "")))
 
+        # Mirror (dual-send) — best-effort second destination
+        raw_mirror = str(v.get("mirror_api_url", ""))
+        if raw_mirror.endswith(_suffix):
+            raw_mirror = raw_mirror[:-len(_suffix)]
+        self.var_mirror_api_url        = tk.StringVar(value=raw_mirror)
+        self.var_mirror_sfm_url        = tk.StringVar(value=str(v.get("mirror_sfm_url", "")))
+        self.var_mirror_sfm_state_file = tk.StringVar(value=str(v.get("mirror_sfm_state_file", "")))
+
     # ── UI construction ───────────────────────────────────────────────────────
 
     def _build_ui(self):
@@ -245,6 +258,7 @@ class SettingsDialog:
         self._build_tab_scanning(nb)
         self._build_tab_logging(nb)
         self._build_tab_forwarding(nb)
+        self._build_tab_mirror(nb)
         self._build_tab_updates(nb)
 
         btn_frame = tk.Frame(outer)
@@ -484,6 +498,49 @@ class SettingsDialog:
         finally:
             self._sfm_test_btn.config(state="normal")
 
+    def _build_tab_mirror(self, nb):
+        f = self._tab_frame(nb, "Mirror")
+
+        intro = (
+            "Optional dual-send: post each heartbeat and event to a SECOND\n"
+            "(\"mirror\") server in addition to the primary. Best-effort — the\n"
+            "mirror can never delay or fail the primary. Leave blank to disable."
+        )
+        tk.Label(f, text=intro, justify="left", fg="#1a5276", wraplength=420).grid(
+            row=0, column=0, columnspan=2, sticky="w", padx=(8, 8), pady=(6, 8)
+        )
+
+        _add_label_entry(f, 1, "Mirror Terra-View URL", self.var_mirror_api_url)
+        _add_label_entry(f, 2, "Mirror SFM URL", self.var_mirror_sfm_url)
+
+        def browse_mirror_state():
+            p = filedialog.asksaveasfilename(
+                title="Select Mirror State File",
+                defaultextension=".json",
+                filetypes=[("JSON files", "*.json"), ("All files", "*.*")],
+                initialfile=os.path.basename(
+                    self.var_mirror_sfm_state_file.get() or "thor_forwarded_mirror.json"),
+                initialdir=os.path.dirname(self.var_mirror_sfm_state_file.get() or "C:\\"),
+            )
+            if p:
+                self.var_mirror_sfm_state_file.set(p.replace("/", "\\"))
+
+        _add_label_browse_entry(f, 3, "Mirror State File", self.var_mirror_sfm_state_file,
+                                browse_mirror_state)
+
+        help_text = (
+            "Mirror Terra-View URL → base like http://10.0.0.x:8001 (heartbeats).\n"
+            "Mirror SFM URL → base like http://10.0.0.x:8200 (events).\n"
+            "The event mirror keeps its OWN state file, so nothing is lost while\n"
+            "the mirror is down — pending events are delivered once it returns.\n"
+            "A quick reachability check skips the mirror cleanly when unreachable,\n"
+            "so the primary is never slowed.\n"
+            "State file blank → defaults to <log_dir>\\thor_forwarded_mirror.json."
+        )
+        tk.Label(f, text=help_text, justify="left", fg="#555555", wraplength=420).grid(
+            row=4, column=0, columnspan=2, sticky="w", padx=(8, 8), pady=(8, 4)
+        )
+
     def _build_tab_updates(self, nb):
         f = self._tab_frame(nb, "Updates")
 
@@ -599,6 +656,12 @@ class SettingsDialog:
             sfm_url = ""
         sfm_url = sfm_url.rstrip("/")  # event_forwarder adds the endpoint path
 
+        # Mirror (dual-send) — best-effort second destination.
+        mirror_api_url = self.var_mirror_api_url.get().strip()
+        if mirror_api_url:
+            mirror_api_url = mirror_api_url.rstrip("/") + "/api/series4/heartbeat"
+        mirror_sfm_url = self.var_mirror_sfm_url.get().strip().rstrip("/")
+
         values = {
             "thordata_path":      self.var_thordata_path.get().strip(),
             "scan_interval":      int_values["Scan Interval"],
@@ -623,6 +686,10 @@ class SettingsDialog:
             "sfm_max_forwards_per_pass":        int_values["Max Forwards Per Pass"],
             "sfm_max_event_age_days":           int_values["Max Event Age (days)"],
             "sfm_state_file":                   self.var_sfm_state_file.get().strip(),
+
+            "mirror_api_url":        mirror_api_url,
+            "mirror_sfm_url":         mirror_sfm_url,
+            "mirror_sfm_state_file":  self.var_mirror_sfm_state_file.get().strip(),
         }
 
         try: