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

api/cron_smart_shopping.php

@@ -79,6 +79,19 @@ try {
         echo '[' . date('Y-m-d H:i:s') . '] Shelf life pre-warm warning: ' . $pe->getMessage() . "\n";
     }
 
+    // ── DB cleanup (retention policy) ────────────────────────────────────
+    // Delete old recipes and transactions based on .env retention settings.
+    try {
+        ob_start();
+        dbCleanup($db);
+        ob_end_clean();
+        echo '[' . date('Y-m-d H:i:s') . '] DB cleanup done'
+            . ' (recipes >' . env('RECIPE_RETENTION_DAYS','7') . 'd'
+            . ', tx >' . env('TRANSACTION_RETENTION_DAYS','7') . 'd' . ")\n";
+    } catch (Throwable $ce) {
+        echo '[' . date('Y-m-d H:i:s') . '] DB cleanup warning: ' . $ce->getMessage() . "\n";
+    }
+
 } catch (Throwable $e) {
     $msg = $e->getMessage();
     echo '[' . date('Y-m-d H:i:s') . '] ERROR: ' . $msg . "\n";