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/messages/de.json

@@ -949,5 +949,60 @@
       "paymentHistory": "Zahlungshistorie",
       "noPayments": "Noch keine Zahlungen vorhanden"
     }
+  },
+  "documents": {
+    "title": "Dokumentenarchiv",
+    "description": "Vereinsdokumente verwalten und archivieren",
+    "upload": "Hochladen",
+    "uploadDocument": "Dokument hochladen",
+    "documentTitle": "Titel",
+    "titlePlaceholder": "z.B. Vereinssatzung 2024",
+    "category": "Kategorie",
+    "selectCategory": "Kategorie wählen",
+    "accessLevel": "Zugriff",
+    "allMembers": "Alle Mitglieder",
+    "boardOnly": "Nur Vorstand",
+    "descriptionLabel": "Beschreibung",
+    "descriptionPlaceholder": "Optionale Beschreibung...",
+    "file": "Datei",
+    "fileHint": "PDF, DOCX, XLSX, PNG, JPG — max. 10 MB",
+    "uploadButton": "Dokument hochladen",
+    "allCategories": "Alle Kategorien",
+    "documentsCount": "Dokumente",
+    "name": "Name",
+    "access": "Zugriff",
+    "size": "Größe",
+    "date": "Datum",
+    "actions": "Aktionen"
+  },
+  "board": {
+    "title": "Vorstand",
+    "description": "Vorstandspositionen und -mitglieder verwalten",
+    "addPosition": "Position anlegen",
+    "electMember": "Mitglied wählen",
+    "createPosition": "Position erstellen",
+    "positionTitle": "Titel",
+    "positionTitlePlaceholder": "z.B. 1. Vorsitzender",
+    "positionDescription": "Beschreibung",
+    "positionDescPlaceholder": "z.B. Gesetzlicher Vertreter gem. §26 BGB",
+    "sortOrder": "Reihenfolge",
+    "save": "Speichern",
+    "electBoardMember": "Vorstandsmitglied wählen",
+    "position": "Position",
+    "selectPosition": "Position wählen",
+    "member": "Mitglied",
+    "selectMember": "Mitglied wählen",
+    "electedAt": "Gewählt am",
+    "termStart": "Amtszeit Beginn",
+    "termEnd": "Amtszeit Ende",
+    "confirmElection": "Wahl bestätigen",
+    "elected": "Gewählt",
+    "term": "Amtszeit",
+    "unlimited": "unbefristet",
+    "termExpiringSoon": "Läuft bald ab",
+    "termActive": "Aktiv",
+    "positions": "Positionen",
+    "active": "Aktiv",
+    "inactive": "Inaktiv"
   }
-}
+}