Initial commit: Arbeitsorte-Logger + data-safety/robustness/input work

Existing app (single-file Tkinter/ttkbootstrap desktop tool for logging
blood-drive work assignments and mapping them) plus the first round of
improvements:

- Data safety: atomic CSV writes (tmp + fsync + os.replace), rotating
  backups in data/backups/ (startup + before every change, keep 20),
  fallback to empty/backup on missing/empty/corrupt orte.csv.
- Geocoder robustness: per-item try/except so the worker thread survives
  failures; GeocodingUnavailable + one-time "service unreachable" dialog.
- Input: DateEntry calendar picker with parse_date() validation; manual
  lat/lon fields in the edit dialog; Tab 2 highlights/filters rows without
  coordinates and adds a right-click "Koordinaten suchen".
- Logging to data/app.log; shared autocomplete helpers; config constants;
  map fit_bounds.

docs/ describes current state, architecture, data model, setup (Linux Mint),
and the full improvement roadmap. data/orte.csv is gitignored for now.

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

.gitignore

@@ -0,0 +1,22 @@
+# Python
+__pycache__/
+*.py[cod]
+.venv/
+venv/
+*.egg-info/
+
+# Generated map (build artefact, regenerated on demand)
+data/karte.html
+
+# Real data — kept out of version control for now (see docs/data-model.md)
+data/orte.csv
+
+# Local backups and logs
+data/backups/
+data/app.log
+data/app.log.*
+
+# Editor / OS
+.vscode/
+.idea/
+.DS_Store