feat: protect postbox message content

tests/test_manifest.py

@@ -10,6 +10,7 @@ from govoplan_core.core.postbox import (
     CAPABILITY_POSTBOX_MESSAGES,
     CAPABILITY_POSTBOX_ROUTING,
 )
+from govoplan_core.core.encryption import CAPABILITY_ENCRYPTION_CONTENT_CIPHER
 from govoplan_postbox.backend.manifest import get_manifest
 
 
@@ -39,6 +40,14 @@ class PostboxManifestTests(unittest.TestCase):
             "idm.function_assignments",
             manifest.required_capabilities,
         )
+        self.assertIn("encryption", manifest.optional_dependencies)
+        self.assertTrue(
+            any(
+                requirement.name == CAPABILITY_ENCRYPTION_CONTENT_CIPHER
+                and requirement.optional
+                for requirement in manifest.requires_interfaces
+            )
+        )
 
 
 if __name__ == "__main__":