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/errors.py

@@ -5,8 +5,22 @@
 """
 
 SCOPE_REASONS = frozenset(
-    {"circuit_open", "retry_exhausted", "stalled", "quota_exhausted", "no_sources"}
+    {
+        "circuit_open",
+        "retry_exhausted",
+        "stalled",
+        "quota_exhausted",
+        "no_sources",
+        "governance_backend_down",  # issue #7: 限流/熔断后端故障(fail-closed → 整个 scope 发不出请求)
+    }
 )
+GOVERNANCE_BACKEND_RETRY_AFTER_S = 5.0
+"""治理后端故障的建议重投间隔(秒)。
+
+**不是环境配置项**——后端恢复时间物理上不可知(不同于熔断冷却有确定到期时刻),
+故取一个保守固定值;下游有自己的退避策略时可忽略本字段。取 0 会让积压任务零延迟
+同时冲击已挂掉的后端,把一次故障放大成一场风暴(issue #7 §3.2)。
+"""
 SOURCE_REASONS = frozenset(
     {
         "network_error",
@@ -127,5 +141,14 @@ class AllSourcesExhausted(GatewayUnavailableError):  # noqa: N818 — ARCH §6.1
     """重试预算耗尽 / 无可用源 / 配额 fail-fast 等 scope 级失败。"""
 
 
+class SourceNotConfiguredError(PolyGatewayError):
+    """源名不在限流后端的配置字典中: 装配缺陷,正常不可达。
+
+    **有意不在** `GatewayUnavailableError` 之下: 它不是"暂时不可用"而是"配置写
+    错了",必须消耗失败预算进死信让人看见;归入可重投家族会让配置错误的任务永远
+    重投、永不告警——正是 issue #7 要修的那个 bug 的镜像(§3.4)。
+    """
+
+
 class GovernanceBackendError(PolyGatewayError):
     """限流/熔断状态后端自身故障: 必须报错而非放行(防击穿网关,降级方向铁律)。"""