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).
- 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>

settings_dialog.py

@@ -53,6 +53,11 @@ DEFAULTS = {
     "SFM_HTTP_TIMEOUT":                 "60",
     "SFM_STATE_FILE":                   "",
     "SFM_MAX_FORWARDS_PER_PASS":        "500",
+
+    # Mirror (dual-send) — best-effort second destination, default OFF.
+    "MIRROR_API_URL":        "",
+    "MIRROR_SFM_URL":        "",
+    "MIRROR_SFM_STATE_FILE": "",
 }
 
 
@@ -260,6 +265,14 @@ class SettingsDialog:
         self.var_sfm_state_file              = tk.StringVar(value=v["SFM_STATE_FILE"])
         self.var_sfm_max_per_pass            = tk.StringVar(value=v["SFM_MAX_FORWARDS_PER_PASS"])
 
+        # Mirror (dual-send) — best-effort second destination
+        _raw_mirror = v["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=v["MIRROR_SFM_URL"])
+        self.var_mirror_sfm_state_file = tk.StringVar(value=v["MIRROR_SFM_STATE_FILE"])
+
     # --- UI construction ---
 
     def _build_ui(self):
@@ -288,6 +301,7 @@ class SettingsDialog:
         self._build_tab_logging(nb)
         self._build_tab_updates(nb)
         self._build_tab_sfm(nb)
+        self._build_tab_mirror(nb)
 
         # Buttons
         btn_frame = tk.Frame(outer)
@@ -566,6 +580,58 @@ class SettingsDialog:
             row=8, column=0, columnspan=2, sticky="w", padx=(8, 8), pady=(8, 4)
         )
 
+    def _build_tab_mirror(self, nb):
+        """Optional dual-send mirror: a second heartbeat + event destination."""
+        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)
+
+        tk.Label(f, text="Mirror State File", anchor="w").grid(
+            row=3, column=0, sticky="w", padx=(8, 4), pady=4
+        )
+        state_frame = tk.Frame(f)
+        state_frame.grid(row=3, column=1, sticky="ew", padx=(0, 8), pady=4)
+        state_frame.columnconfigure(0, weight=1)
+        ttk.Entry(state_frame, textvariable=self.var_mirror_sfm_state_file, width=32).grid(
+            row=0, column=0, sticky="ew"
+        )
+
+        def _browse_mirror_state():
+            path = filedialog.asksaveasfilename(
+                title="Mirror forward-state file",
+                defaultextension=".json",
+                filetypes=[("JSON", "*.json"), ("All Files", "*.*")],
+                initialfile="sfm_forwarded_mirror.json",
+            )
+            if path:
+                self.var_mirror_sfm_state_file.set(path)
+
+        ttk.Button(state_frame, text="Browse...", width=10,
+                   command=_browse_mirror_state).grid(row=0, column=1, padx=(4, 0))
+
+        hint_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\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>/sfm_forwarded_mirror.json."
+        )
+        tk.Label(f, text=hint_text, justify="left", fg="#555555", wraplength=420).grid(
+            row=4, column=0, columnspan=2, sticky="w", padx=(8, 8), pady=(8, 4)
+        )
+
     def _test_sfm_connection(self):
         """GET <sfm_url>/health and show the result."""
         import urllib.request
@@ -673,6 +739,12 @@ class SettingsDialog:
         else:
             api_url = api_url.rstrip("/") + "/api/series3/heartbeat"
 
+        # 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/series3/heartbeat"
+        mirror_sfm_url = self.var_mirror_sfm_url.get().strip().rstrip("/")
+
         values = {
             "API_ENABLED":          "true" if self.var_api_enabled.get() else "false",
             "API_URL":              api_url,
@@ -698,6 +770,10 @@ class SettingsDialog:
             "SFM_HTTP_TIMEOUT":                 str(int_values["SFM HTTP Timeout"]),
             "SFM_STATE_FILE":                   self.var_sfm_state_file.get().strip(),
             "SFM_MAX_FORWARDS_PER_PASS":        str(int_values["SFM Max Events Per Pass"]),
+
+            "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: