fix: dispensa.db auto-delete, zerowaste save, vacuum expiry, DB retention

- api/index.php: auto-delete legacy dispensa.db when evershelf.db exists
  and dispensa.db is empty (<1KB); vacuum-sealed items only show as
  expired after VACUUM_EXPIRY_EXTENSION_DAYS (default 30) past printed
  date; add dbCleanup() function; add recipe/tx/vacuum params to
  getServerSettings + saveSettings intMap; add 'db_cleanup' action
- api/cron_smart_shopping.php: run dbCleanup() each cron cycle
- app.js: add zerowaste_tips_enabled + screensaver_timeout + retention
  days to saveSettings POST payload (were missing, causing reset on sync);
  asset version bumped to v=20260518b
- .env: added ZEROWASTE_TIPS_ENABLED, RECIPE_RETENTION_DAYS=7,
  TRANSACTION_RETENTION_DAYS=7, VACUUM_EXPIRY_EXTENSION_DAYS=30

index.html

@@ -1652,6 +1652,6 @@
     </div>
 </div>
 
-<script src="assets/js/app.js?v=20260518a"></script>
+<script src="assets/js/app.js?v=20260518b"></script>
 </body>
 </html>