feat: serve isolated companies below a fixed product path

Assistant: codex
Assistant-Model: gpt-6-astra
Assistant-Session: 01a07ff8-19d0-7820-b4d0-1353833cb7fc

vergabe_teilnahme/settings/paths.py

@@ -0,0 +1,12 @@
+import re
+
+from django.core.exceptions import ImproperlyConfigured
+
+
+def application_path(value):
+    """A deployment-owned tenant path, never a value taken from request headers."""
+    if value in ('', '/'):
+        return ''
+    if not re.fullmatch(r'/[a-z0-9]+(?:-[a-z0-9]+)*', value):
+        raise ImproperlyConfigured('APP_BASE_PATH must be empty or /lowercase-tenant-slug')
+    return value