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/setup.md

@@ -7,16 +7,24 @@ during install.
 ## 1. System packages
 
 Tkinter is not bundled with the system Python on Mint and must be installed
-separately:
+separately. `git` is needed for the in-app updater:
 
 ```bash
 sudo apt update
-sudo apt install python3-tk python3-venv python3-pip
+sudo apt install python3-tk python3-venv python3-pip git
 ```
 
 ## 2. Get the code
 
-Put the project folder somewhere stable, e.g. `~/Apps/DRK_Blutspende_Orte`.
+**Clone it** (don't download a zip) – the in-app "Nach Update suchen" only works
+from a git clone:
+
+```bash
+mkdir -p ~/Apps && cd ~/Apps
+git clone <repo-url> DRK_Blutspende_Orte
+```
+
+Put it somewhere stable, e.g. `~/Apps/DRK_Blutspende_Orte`.
 
 ## 3. Create a virtual environment and install dependencies
 
@@ -94,13 +102,26 @@ that. The macOS CommandLineTools Python used during development ships Tk 8.5 and
 
 ## Updating
 
+**From inside the app:** menu **Hilfe ▸ Nach Update suchen**. The app also
+checks quietly on start and offers the update if there is one. It fast-forwards
+to the server version, reinstalls dependencies if `requirements.txt` changed,
+and restarts itself. `data/` is never touched.
+
+For this to work the app must run from a **git clone** (step 2) whose `.venv`
+was made with the same Python it runs on, and `git` must be installed. The
+updater only ever fast-forwards – if the local copy has been changed by hand it
+refuses and says to get in touch.
+
+**Manually** (equivalent):
+
 ```bash
 cd ~/Apps/DRK_Blutspende_Orte
-git pull            # once the project is in git
+git pull --ff-only
 .venv/bin/pip install -r requirements.txt
 ```
 
 ## Data location
 
 All state is in `data/orte.csv` next to `app.py`. To back up the app, copy that
-one file. To move to a new laptop, copy the whole folder and redo steps 1 & 3.
+one file (plus `data/backups/` if you want the history). To move to a new
+laptop, clone the repo again and redo steps 1 & 3; copy `data/orte.csv` across.