i18n: replace all hardcoded Italian strings with English

- api/index.php: health check hints (disk space, DB, Gemini, TTS, scale,
  internet) translated to English; Bring! error strings (credentials,
  list not found, fetch error, missing params) translated; Gemini chat
  and identify_product error strings translated; transaction note
  [Correzione manuale] -> [Manual correction]
- assets/js/app.js: expiry scanner result strings now use t() keys
  (scanner.expiry_found, scanner.expiry_read_fail, scanner.expiry_raw_label);
  removed Italian fallback from kiosk native_update_hint toast
- translations/{it,en,de}.json: added scanner.expiry_found,
  scanner.expiry_read_fail, scanner.expiry_raw_label keys
- README.md: 'Generali' tab label -> 'General' (2 occurrences)

translations/en.json

@@ -1034,7 +1034,10 @@
     "retake_btn": "🔄 Retake",
     "camera_error_hint": "Ensure you use HTTPS and have granted camera permissions.<br>You can enter the barcode manually or use AI identification.",
     "no_barcode": "No barcode",
-    "save_new_btn": "🆕 None of these — save as new"
+    "save_new_btn": "🆕 None of these — save as new",
+    "expiry_found": "Date found",
+    "expiry_read_fail": "Cannot read the date.",
+    "expiry_raw_label": "Read"
   },
   "lowstock": {
     "title": "⚠️ Running low!",