feat: initialize governed postbox module

tests/test_module_boundary.py

@@ -0,0 +1,37 @@
+from __future__ import annotations
+
+import ast
+import unittest
+from pathlib import Path
+
+
+class PostboxModuleBoundaryTests(unittest.TestCase):
+    def test_backend_does_not_import_sibling_module_implementations(self) -> None:
+        root = Path(__file__).parents[1] / "src" / "govoplan_postbox"
+        forbidden = (
+            "govoplan_access",
+            "govoplan_campaign",
+            "govoplan_files",
+            "govoplan_identity",
+            "govoplan_idm",
+            "govoplan_mail",
+            "govoplan_organizations",
+            "govoplan_portal",
+        )
+        violations: list[str] = []
+        for path in root.rglob("*.py"):
+            tree = ast.parse(path.read_text(encoding="utf-8"), filename=str(path))
+            for node in ast.walk(tree):
+                names: list[str] = []
+                if isinstance(node, ast.Import):
+                    names.extend(alias.name for alias in node.names)
+                elif isinstance(node, ast.ImportFrom) and node.module:
+                    names.append(node.module)
+                for name in names:
+                    if name.startswith(forbidden):
+                        violations.append(f"{path.relative_to(root)}: {name}")
+        self.assertEqual([], violations)
+
+
+if __name__ == "__main__":
+    unittest.main()