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/dokumente.py

@@ -31,6 +31,7 @@ class DokumentDatei(models.Model):
         ("korrespondenz", "Korrespondenz / Brief"),
         ("bescheid", "Bescheid / Behörde"),
         ("stiftungsgeschichte", "Stiftungsgeschichte / Archiv"),
+        ("email", "E-Mail-Nachricht"),
         ("anderes", "Sonstiges"),
     ]