✨(backend) add server-to-server API endpoint to create documents

We want trusted external applications to be able to create documents
via the API on behalf of any user. The user may or may not pre-exist
in our database and should be notified of the document creation by
email.

src/backend/core/authentication/__init__.py

@@ -0,0 +1,52 @@
+"""Custom authentication classes for the Impress core app"""
+
+from django.conf import settings
+
+from rest_framework.authentication import BaseAuthentication
+from rest_framework.exceptions import AuthenticationFailed
+
+
+class ServerToServerAuthentication(BaseAuthentication):
+    """
+    Custom authentication class for server-to-server requests.
+    Validates the presence and correctness of the Authorization header.
+    """
+
+    AUTH_HEADER = "Authorization"
+    TOKEN_TYPE = "Bearer"  # noqa S105
+
+    def authenticate(self, request):
+        """
+        Authenticate the server-to-server request by validating the Authorization header.
+
+        This method checks if the Authorization header is present in the request, ensures it
+        contains a valid token with the correct format, and verifies the token against the
+        list of allowed server-to-server tokens. If the header is missing, improperly formatted,
+        or contains an invalid token, an AuthenticationFailed exception is raised.
+
+        Returns:
+            None: If authentication is successful
+                  (no user is authenticated for server-to-server requests).
+
+        Raises:
+            AuthenticationFailed: If the Authorization header is missing, malformed,
+            or contains an invalid token.
+        """
+        auth_header = request.headers.get(self.AUTH_HEADER)
+        if not auth_header:
+            raise AuthenticationFailed("Authorization header is missing.")
+
+        # Validate token format and existence
+        auth_parts = auth_header.split(" ")
+        if len(auth_parts) != 2 or auth_parts[0] != self.TOKEN_TYPE:
+            raise AuthenticationFailed("Invalid authorization header.")
+
+        token = auth_parts[1]
+        if token not in settings.SERVER_TO_SERVER_API_TOKENS:
+            raise AuthenticationFailed("Invalid server-to-server token.")
+
+        # Authentication is successful, but no user is authenticated
+
+    def authenticate_header(self, request):
+        """Return the WWW-Authenticate header value."""
+        return f"{self.TOKEN_TYPE} realm='Create document server to server'"