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/views/__init__.py

@@ -206,5 +206,12 @@ from .veranstaltung import (  # noqa: F401
     teilnehmer_delete,
 )
 
+from .import_export import (  # noqa: F401
+    import_export_hub,
+    csv_export,
+    csv_import_upload,
+    csv_import_execute,
+)
+
 # Non-view exports (helpers used elsewhere)
 from .system import GrampsClient, get_gramps_client, get_pdf_generator  # noqa: F401