chore: initialize from backend template 0a6dd0e

docs/runbooks/db-deadlock.md

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+---
+title: Runbook — DB_DEADLOCK (데드락)
+category: CONFLICT
+error_codes: [DB_DEADLOCK]
+severity: P2
+owner: oncall
+last_updated: 2026-06-15
+status: stub
+---
+
+# Runbook: DB_DEADLOCK (`runbook://db/deadlock`)
+
+## Symptoms
+
+- HTTP 409 with `error.code=DB_DEADLOCK`
+- SQLState 40P01 in Postgres logs
+
+## Diagnosis
+
+- Check `pg_locks` and `pg_stat_activity` during deadlock
+- Identify conflicting transaction lock order
+
+## Action
+
+- Client should retry (retryable=true)
+- Fix lock ordering in code if recurring
+
+## Escalation
+
+- Escalate to DBA if deadlock rate is sustained > 1% of transactions
+
+---
+
+> **Stub**: Phase D2 — author body after domain adoption. (feature-operational-runbook-contract D9)