Add in-app self-update via git (Hilfe menu)

New Updater class + "Hilfe" menubar:
- "Nach Update suchen": git fetch + count of new upstream commits.
- Quiet background check on start; surfaces via status bar + menu label.
- Install: git merge --ff-only, pip install if requirements.txt changed,
  then restart via os.execv. data/ is git-ignored and untouched.
- Fast-forward only; diverged history or offline -> clear message, no action.
- Inert unless run from a git clone with git on PATH.

Also: "Version…" menu item shows the installed commit.

Verified against throwaway git repos (check / ff-update / diverged / no-op /
non-clone) and via GUI build on Python 3.14 / Tk 9.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/changelog.md

@@ -2,6 +2,26 @@
 
 Notable changes to the app. Newest first.
 
+## 2026-09-07 — built-in self-update
+
+New **Hilfe** menu:
+
+- **Nach Update suchen** – `git fetch` + count of new commits on the server.
+- **Version…** – shows the installed commit (`<hash> · <date>`).
+
+On start, a quiet background check runs; if updates exist the status bar says so
+and the menu item becomes "Update installieren (N verfügbar)".
+
+Installing runs `git merge --ff-only` (fast-forward only – never an automatic
+merge), re-runs `pip install -r requirements.txt` if that file changed, then
+restarts the app via `os.execv`. User data (`data/`) is git-ignored and
+untouched. If the local checkout has diverged or the network is down, it shows a
+clear message and does nothing.
+
+Only active when the app runs from a **git clone** with `git` installed;
+otherwise the menu just shows "Version: unbekannt". New `Updater` class,
+`app.py`.
+
 ## 2026-09-07 — map tiles switched to OpenStreetMap
 
 `folium.Map(tiles="CartoDB positron")` began showing an "API KEY REQUIRED"