fix: TTS test ask user if heard + PHP 8 end() ref bug + DB migration guard for fresh volumes

- app.js: TTS kiosk timeout 4s → 10s; fires interactive 'Hai sentito?' YES/NO
  prompt instead of showing error (TTS can take 6-8s; UtteranceProgressListener
  may not fire on all firmware); YES → success, NO → troubleshooting steps
- translations: add heard_question/heard_yes/heard_no/test_ok_kiosk/test_fail_steps
  to all 5 languages (it/en/de/fr/es) under settings.tts
- api/index.php: fix end() PHP 8.0+ reference error in _offFetchProduct()
  (categories_hierarchy stored in temp var before calling end())  (fixes #130)
- api/database.php: migrateDB() now checks sqlite_master for 'products' table;
  if missing, calls initializeDB() and returns — no ALTER on nonexistent table
  (fixes #133, covers #131)
- api/index.php: health_check db_row_count query guarded against missing
  inventory table (fixes #131)

translations/en.json

@@ -701,7 +701,12 @@
             "voices_hint": "Available voices depend on the OS and browser. On macOS/iOS the Paola (Italian) voice is available. Press ↺ if the list does not load.",
             "url_missing": "⚠️ Endpoint URL missing.",
             "test_sending": "⏳ Sending…",
-            "test_ok": "✅ Response {code} — check that the speaker has spoken."
+            "test_ok": "✅ Response {code} — check that the speaker has spoken.",
+            "heard_question": "Did you hear the voice?",
+            "heard_yes": "Yes, I heard it",
+            "heard_no": "No, I didn't hear it",
+            "test_ok_kiosk": "TTS is working.",
+            "test_fail_steps": "Check: 1) media volume is not 0; 2) Google Text-to-Speech is installed and updated; 3) Italian voice package is downloaded in Android TTS settings."
         },
         "language": {
             "title": "🌐 Language",