feat: scan history browser, user-scoped viewer tokens, export fixes, email fixes (v1.6.20)

- Scan history browser (history.js, GET /api/db/sessions, get_sessions(),
  get_session_items(ref_scan_id)) — review any past session without rescanning
- User-scoped viewer tokens (#34) — scope by individual employee across M365
  and GWS; autocomplete from Accounts list; dual-email support
- Fix: GWS scan never marked finished (end_scan → finish_scan) and emitted
  wrong SSE event (scan_done → google_scan_done), excluding GWS items from all
  exports
- Fix: file scan begin_scan called with wrong keyword args (TypeError swallowed),
  so local/SMB items were never written to DB
- Fix: Graph sendMail reported failure on success — _post() now returns {} on
  empty 202 response instead of raising JSONDecodeError
- Fix: Graph error hidden behind generic "No SMTP host" message when both Graph
  and SMTP were unavailable
- Fix: Gmail vs Google Workspace SMTP error messages distinguished by username
  domain; Workspace errors point to admin console, not personal security settings
- Docs: update README, MANUAL-EN, MANUAL-DA, CLAUDE.md, TODO.md, CHANGELOG.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lang/de.json

@@ -164,6 +164,13 @@
   "lbl_working": "Wird bearbeitet…",
   "lbl_stopping": "Wird gestoppt…",
   "lbl_loading": "Wird geladen…",
+  "history_lbl": "Verlauf",
+  "history_items": "Treffer",
+  "history_btn_sessions": "Sessionen",
+  "history_btn_latest": "Letzter Scan",
+  "history_picker_empty": "Keine fr\u00fcheren Scans",
+  "history_delta_badge": "Delta",
+  "history_latest_badge": "Aktuell",
   "lbl_blurred": "Unscharf gemacht",
   "lbl_none": "Keine",
   "lbl_size": "Größe",
@@ -766,8 +773,14 @@
   "share_create_error": "Link konnte nicht erstellt werden:",
   "share_revoke_confirm": "Diesen Link widerrufen? Alle Nutzer verlieren sofort den Zugriff.",
   "share_revoke_error": "Widerrufen fehlgeschlagen:",
-  "share_scope_lbl": "Rollenumfang",
-  "share_scope_all": "Alle Rollen",
+  "share_scope_lbl": "Bereich",
+  "share_scope_all": "Alle",
+  "share_scope_type_role": "Rolle",
+  "share_scope_type_user": "Benutzer",
+  "share_scope_role_lbl": "Rolle",
+  "share_scope_user_lbl": "Benutzer-E-Mail",
+  "share_scope_user_placeholder": "alice@schule.de",
+  "share_scope_user_invalid": "Bitte gib eine gültige E-Mail-Adresse für den Benutzerbereich an.",
   "share_scope_staff": "Mitarbeitende",
   "share_scope_student": "Schüler",