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.
Includes config aggregation for multi-source env keys, from_env and
from_settings factories with explicit shared-backend injection,
gather_bounded, top-level exports, tightened import-linter layers with
the gate removed from the Makefile, and the finalized .env.example.
iomgaa