fix(updater): keep auto-updater alive across watcher restarts (v0.4.1)

Saving the Settings dialog calls _restart_watcher, which set+replaced the
shared stop_event the updater loop also keyed off — so a settings save
silently killed the auto-updater thread (race: survived one save, died on
the next). Heartbeats kept flowing but the agent stopped checking for /
applying updates until relaunch.

Give the updater its own app-lifetime event (_app_stop), set only on
Exit / when applying an update and untouched by watcher restarts. Adds
test_thor_tray.py (4 lifecycle tests). Bump to v0.4.1.

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

README.md

@@ -1,6 +1,6 @@
 # Thor Watcher
 
-**Version:** 0.4.0
+**Version:** 0.4.1
 
 Micromate (Series 4) watcher agent for Terra-View fleet management. Runs as a Windows system tray application, scans THORDATA for Micromate unit activity, sends heartbeat data to Terra-View, and (optionally) forwards `.IDFH`/`.IDFW` event files to a seismo-relay SFM server. Heartbeats and events can optionally be **mirrored** to a second server (e.g. a standby/NAS) in addition to the primary.
 
@@ -29,7 +29,7 @@ build.bat
 ```
 
 Produces:
-- `dist\thor-watcher-0.4.0.exe` — upload to Gitea release
+- `dist\thor-watcher-0.4.1.exe` — upload to Gitea release
 - `dist\thor-watcher.exe` — use with Inno Setup
 
 Then run Inno Setup Compiler on `installer.iss` to produce `thor-watcher-setup.exe`.
@@ -139,7 +139,7 @@ Posted to `api_url` on each API interval:
 {
   "source_id": "THOR-PC",
   "source_type": "series4_watcher",
-  "version": "0.4.0",
+  "version": "0.4.1",
   "generated_at": "2026-03-20T14:30:00Z",
   "log_tail": ["...last 25 log lines..."],
   "units": [