fix: reparent governance backend failures under GatewayUnavailableError (issue #7)

A fail-closed limiter or breaker backend means the scope cannot emit a
single request, which is exactly scope-level unavailability. But the error
sat directly under PolyGatewayError, so a caller writing only
`except GatewayUnavailableError` dropped it into the catch-all branch:
Redis blips once and a backlog of tasks burns its business failure budget
into the dead letter queue, over a fault a restart would clear.

Three gate paths leak to callers rather than being absorbed by
_record_quietly (try_acquire, try_enter, progress_age_s); each is now
pinned by a test, since none of them had one before.

The two unknown-source sites move to SourceNotConfiguredError instead of
following along. They report a misconfigured source name, not an outage,
and letting them into the retryable family would be the mirror of the bug
being fixed here: the task would retry forever and never surface.

src/polygateway/middleware/retry.py

@@ -183,8 +183,8 @@ class RetryMW:
         self._scope = scope
         self._sources = list(sources)
         self._selector = selector
-        self._quota = QuotaGate(limiter)
-        self._breaker = BreakerGate(gate)
+        self._quota = QuotaGate(limiter, scope=self._scope)
+        self._breaker = BreakerGate(gate, scope=self._scope)
         self._transport = transport
         self._retry = retry
         self._bp = backpressure