v4.1.0: DMS email documents, category-specific Nachweis linking, version system

- Save cover email body as DMS document with new 'email' context type
- Show email body separately from attachments in email detail view
- Add per-category DMS document assignment in quarterly confirmation
  (Studiennachweis, Einkommenssituation, Vermögenssituation)
- Add VERSION file and context processor for automatic version display
- Add MCP server, agent system, import/export, and new migrations
- Update compose files and production environment template

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

app/stiftung/models/system.py

@@ -11,6 +11,10 @@ class CSVImport(models.Model):
         ("paechter", "Pächter"),
         ("laendereien", "Ländereien"),
         ("verpachtungen", "Verpachtungen"),
+        ("foerderungen", "Förderungen"),
+        ("konten", "Stiftungskonten"),
+        ("verwaltungskosten", "Verwaltungskosten"),
+        ("rentmeister", "Rentmeister"),
         ("personen", "Personen (Legacy)"),
     ]
 
@@ -111,6 +115,8 @@ class ApplicationPermission(models.Model):
             # System Permissions
             ("access_django_admin", "Kann Django Admin aufrufen"),
             ("view_system_stats", "Kann Systemstatistiken anzeigen"),
+            # AI Agent Permissions
+            ("can_use_agent", "Kann AI-Assistenten nutzen"),
         ]