refactor(runner): 收敛上游失败决策与轮转策略

将同平台重试、跨平台动作、健康副作用和冷却排队统一到单一失败决策，避免旧降级策略与 failover 重复执行。\n\n增加事务级单源保护、候选实时刷新、冷却错误契约、管理接口严格校验及兼容策略只读展示。\n\n验证：真实 Gateway HTTP/PostgreSQL 接受测试 10 项通过；go test ./...、pnpm openapi、pnpm lint、pnpm test、pnpm build 均通过。

apps/api/internal/runner/trace.go

@@ -48,6 +48,29 @@ func failoverTraceEntry(decision failoverDecision, candidate store.RuntimeModelC
 	return entry
 }
 
+func failureDecisionTraceEntry(decision failureDecision, candidate store.RuntimeModelCandidate, clientAttempt int, maxAttempts int, applied bool, guardReason string) map[string]any {
+	entry := policyTraceEntry("failure_decision", "candidate_failure", decision.Route, decision.Reason, decision.Match, decision.Info)
+	entry["route"] = decision.Route
+	entry["effect"] = decision.Effect
+	entry["applied"] = applied
+	entry["guardReason"] = guardReason
+	entry["clientAttempt"] = clientAttempt
+	entry["maxAttempts"] = maxAttempts
+	entry["rule"] = decision.Match.Rule
+	entry["value"] = decision.Match.Value
+	if decision.Target != "" {
+		entry["target"] = decision.Target
+	}
+	if decision.CooldownSeconds > 0 {
+		entry["cooldownSeconds"] = decision.CooldownSeconds
+	}
+	if decision.DemoteSteps > 0 {
+		entry["demoteSteps"] = decision.DemoteSteps
+	}
+	addCandidatePriorityTraceFields(entry, candidate)
+	return entry
+}
+
 func priorityDemoteTraceEntry(decision priorityDemoteDecision, platformID string, platformModelID string, dynamicPriority int) map[string]any {
 	entry := policyTraceEntry("priority_demoted", "priority_demote", "demote", decision.Reason, decision.Match, decision.Info)
 	entry["demote"] = decision.Demote