chore: initialize from backend template 0a6dd0e

docs/runbooks/dependency-5xx-server.md

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+---
+title: Runbook — DEPENDENCY_5XX_SERVER (업스트림 서버 오류)
+category: TRANSIENT_DEPENDENCY
+error_codes: [DEPENDENCY_5XX_SERVER]
+severity: P1
+owner: oncall
+last_updated: 2026-06-15
+status: stub
+---
+
+# Runbook: DEPENDENCY_5XX_SERVER (`runbook://dependency/5xx-server`)
+
+## Symptoms
+
+- HTTP 502 with `error.code=DEPENDENCY_5XX_SERVER`
+- Upstream returned 5xx; transient server-side failure
+
+## Diagnosis
+
+- Check `dependency_name` tag for which upstream is failing
+- Review upstream service status page
+
+## Action
+
+- Client should retry (retryable=true)
+- Monitor upstream recovery
+
+## Escalation
+
+- P1 if critical upstream is in sustained 5xx state
+
+---
+
+> **Stub**: Phase D2 — author body after domain adoption. (feature-operational-runbook-contract D9)