add merge tool

README.md

@@ -55,6 +55,19 @@ Or use the preconfigured VSCode tasks (`Ctrl+Shift+B`) and launch configurations
 
 Tested with: [Epson TM T20II](https://www.epson.de/products/sd/pos-printer/epson-tm-t20ii). Other receipt printers should work as well.
 
+## Tools
+### merge_db
+
+Merges multiple SQLite databases into one file with automatic deduplication.
+Reference tables (`category`, `positionen`) are deduplicated by content; `jobs` rows are appended in order with re-sequenced IDs to avoid conflicts.
+
+```bash
+python tools/merge_db.py -o OUTPUT_DB INPUT_DB [INPUT_DB ...]
+
+# Example
+python tools/merge_db.py -o merged.db 02.db 03.db 04.db
+```
+
 ## License
 
 GPL-3.0