feat(sprint8): Phase 4 — Dokumentenarchiv + Vorstandsverwaltung

Backend:
- V20 migration: documents table with category, access_level, file storage
- V21 migration: board_positions + board_members with term tracking
- Document entity + DocumentCategory/DocumentAccessLevel enums
- BoardPosition + BoardMember entities
- Extended AuditEventType (DOCUMENT_UPLOADED/DELETED, BOARD_MEMBER_ELECTED/REMOVED)
- Extended StaffPermission (MANAGE_DOCUMENTS)
- Extended NotificationType (BOARD_TERM_EXPIRING)
- DocumentService: upload, list, download, delete, storage usage
- BoardService: positions CRUD, elect/remove members, current/history
- DocumentController: multipart upload, filtered list, download, delete, portal
- BoardController: positions, elect, remove, current board, history, portal

Frontend:
- documents.ts + board.ts service layers
- Admin /documents page: grouped by category, upload dialog, filter, download/delete
- Admin /board page: current board cards, position management, elect member dialog
- Navigation: added Dokumente + Vorstand to sidebar
- i18n: documents.* + board.* keys in de.json + en.json

cannamanage-frontend/src/data/navigations.ts

@@ -49,6 +49,16 @@ export const navigationsData: NavigationType[] = [
         href: "/assemblies",
         iconName: "Gavel",
       },
+      {
+        title: "Dokumente",
+        href: "/documents",
+        iconName: "FileArchive",
+      },
+      {
+        title: "Vorstand",
+        href: "/board",
+        iconName: "Shield",
+      },
       {
         title: "Kalender",
         href: "/calendar",