Add lot_prefix to products; compose LOT number from prefix + date suffix

Products now have a configurable lot_prefix (e.g. "112"). In the batch form
the prefix is shown read-only next to the suffix input (JJMMTT), and the full
lot_number (e.g. "112-260912") is composed automatically and stored.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -110,7 +110,7 @@ GET https://api.sms.spices.blue/api/v1/{lot_number}
 ```json
 {
   "data": {
-    "lot_number": "LOT-2024-001",
+    "lot_number": "112-260912",
     "produced_at": "2024-10-01",
     "expires_at":  "2026-10-01",
     "units_produced": 50,
@@ -220,6 +220,7 @@ Table packings {
 Table products {
     id int [pk]
     title text
+    lot_prefix varchar       -- Präfix für Chargennummer, z.B. "112"
     images json
     description text
     recipes_id int [ref: > recipes.id]