Show all open (unactioned) items by default, not just the last scan

The default results view loaded only the latest scan session (±300s
window), so items dropped out of sight once a newer scan started — and
a long scheduled scan could show little or nothing on browser open.

Add get_open_items(): every flagged item with no disposition (or status
'unreviewed') across all scans, deduped by id to the latest finished
scan. GET /api/db/flagged now serves it when no ?ref is given; ?ref=N
still loads a specific past session. Frontend loadHistorySession(null)
routes to a new loadOpenItems() loader. Rename the banner button to
"Open items" (da/de/en).

get_session_items() default is unchanged — export.py and
scan_scheduler.py still rely on latest-session for the current scan's
report/email.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lang/de.json

@@ -167,7 +167,7 @@
   "history_lbl": "Verlauf",
   "history_items": "Treffer",
   "history_btn_sessions": "Sessionen",
-  "history_btn_latest": "Letzter Scan",
+  "history_btn_latest": "Offene Einträge",
   "history_picker_empty": "Keine früheren Scans",
   "history_delta_badge": "Delta",
   "history_latest_badge": "Aktuell",