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>

docs/overview.md

@@ -0,0 +1,85 @@
+# Overview – what the app does
+
+The app is a single window with a status bar at the bottom and two tabs.
+
+## Tab 1 – "Neuer Eintrag" (new entry)
+
+Workflow: build up a **queue** of entries, let them geocode in the background,
+then save the whole batch at once.
+
+1. **Entry form** – date (a **calendar picker**, `ttkbootstrap.DateEntry`,
+   pre-filled with today; typed input is accepted in `JJJJ-MM-TT` or
+   `TT.MM.JJJJ` and **validated** – an invalid date is rejected with a dialog),
+   city (autocomplete combobox), postal code (optional). Pressing `Return` in any
+   field, or the **+ Hinzufügen** button, adds the row to the queue.
+2. **Queue table** – shows date, city, PLZ, and a live coordinate column that
+   updates from `⏳ wird gesucht…` to either `lat / lon` or `⚠ nicht gefunden`
+   as the background geocoder works through the queue.
+   - Left-click the `✕` cell to remove a row.
+   - Right-click a row for *Löschen* / *Koordinaten erneut suchen*.
+3. **Action bar**
+   - **Alle speichern** – appends every queued row to `orte.csv`. If a row looks
+     like a duplicate of an existing entry (same date + city + PLZ) a hint is
+     shown in the status bar, but the row is **still saved**.
+   - **Leeren** – discards the queue without saving.
+   - **Karte öffnen** – regenerates `karte.html` and opens it in the browser.
+
+## Tab 2 – "Einträge verwalten" (manage entries)
+
+A table view of everything in `orte.csv`.
+
+- **Search box** – live filter across date, city, and PLZ (substring match).
+- **Sortable columns** – click a header to sort; clicking again reverses.
+  Default sort is by date, newest first.
+- **"Nur ohne Koordinaten"** toggle – filters to entries that have no
+  coordinates yet. Such rows are also shown in **red** in the full list, and the
+  hint line reports how many there are.
+- **Edit** – double-click a row (or *Bearbeiten*) opens a modal dialog to change
+  date (calendar picker, validated) / city / PLZ. Two ways to fix coordinates:
+  a "Koordinaten automatisch neu suchen" toggle re-runs geocoding after saving,
+  or the **Breitengrad / Längengrad** fields let you type them in by hand
+  (both empty = no map marker).
+- **Right-click → Koordinaten suchen** – runs geocoding for that one row
+  (handy for rows that failed the first time).
+- **Delete** – *Löschen* removes the selected row(s) after a confirmation
+  dialog. This is **irreversible**, but a timestamped copy of the file is
+  written to `data/backups/` before every change (see
+  [data-model.md](data-model.md#backups)).
+- **Karte öffnen** – same as on Tab 1.
+
+## The map (`karte.html`)
+
+Generated by `generate_map()`:
+
+- Base layer: `CartoDB positron`, initial view centred on `[49.0, 9.0]`,
+  zoom 8 (roughly Baden-Württemberg).
+- One **`CircleMarker`** per unique `(city, lat, lon)` group. Radius scales
+  linearly with the visit count for that location
+  (`5 + count / max_count * 20` px), colour is DRK red (`#CC0000`).
+- Popup shows the city and the visit count; tooltip shows the city.
+- Rows with missing/blank coordinates are silently excluded.
+- If there are no usable coordinates at all, a `ValueError` is raised and shown
+  in the status bar.
+
+> The generated HTML pulls Leaflet, jQuery and Bootstrap from CDNs, so the map
+> needs an internet connection to render even though the data is local.
+
+## Geocoding behaviour
+
+`geocode_city(city, postal_code)` tries a series of queries in order and returns
+the first hit:
+
+1. `"<PLZ> <city>, Germany"` (only if a PLZ was given)
+2. `"<city>, Baden-Württemberg, Germany"`
+3. `"<city>, Hessen, Germany"`
+4. `"<city>, Germany"`
+
+The regional bias is the `GEOCODE_REGIONS` constant (`Baden-Württemberg`,
+`Hessen`) near the top of `app.py`. Results are rounded to five decimal places.
+Nominatim's usage policy (max 1 req/s, identifying `user_agent`) is respected by
+a manual `time.sleep(1)` in the worker loop.
+
+If the geocoding service is unreachable (no internet), entries still save –
+without coordinates – and after a few consecutive failures a one-time dialog
+explains that the coordinates can be added later via *Bearbeiten*. The worker
+thread logs failures to `data/app.log` and keeps running.