feat: add SourceNotConfiguredError and the governance backend reason

Pure addition ahead of the reparenting, so this commit leaves every
existing caller and test untouched.

SourceNotConfiguredError deliberately stays outside GatewayUnavailableError:
a source name that is not in the limiter's config dict is an assembly
defect, not a transient outage, and folding it into the retryable family
would let a typo retry forever without ever reaching a dead letter queue.

The retry_after_s default is 5.0 rather than 0 because a backlog released
at zero delay would stampede a backend that is already down.

src/polygateway/__init__.py

@@ -17,6 +17,7 @@ from polygateway.errors import (
     RequestRejectedError,
     ResultInvalidError,
     SourceDeadError,
+    SourceNotConfiguredError,
     TransientError,
 )
 from polygateway.ocr import OcrClient
@@ -58,6 +59,7 @@ __all__ = [
     "ResultInvalidError",
     "SourceConfig",
     "SourceDeadError",
+    "SourceNotConfiguredError",
     "TransientError",
     "__version__",
     "gather_bounded",