test(12-05): add migration 0005→0006 integration regression and RUNBOOK migration ops

- test_migration_0006.py: proves display_name_override, cloud_items, cloud_item_topics,
  cloud_folder_states, unique constraints, indexes, and DDL privilege boundary
- Tests skip cleanly without INTEGRATION=1/INTEGRATION_DATABASE_URL (no dev DB mutation)
- RUNBOOK: add Database Migrations section with revision check, upgrade, recovery, and
  emergency schema-drift remediation commands

RUNBOOK.md

@@ -13,12 +13,13 @@ for phase history and roadmap see `.planning/ROADMAP.md`.
 
 1. [Environment Variables](#1-environment-variables)
 2. [Startup / Shutdown](#2-startup--shutdown)
-3. [Backup Strategy](#3-backup-strategy)
-4. [Health Checks](#4-health-checks)
-5. [Security Gate — docker scout CVE Scan (D-10)](#5-security-gate--docker-scout-cve-scan-d-10)
-6. [On-Call Escalation](#6-on-call-escalation)
-7. [Common Failure Modes](#7-common-failure-modes)
-8. [Phase 6 Deferred Items](#8-phase-6-deferred-items)
+3. [Database Migrations](#3-database-migrations)
+4. [Backup Strategy](#4-backup-strategy)
+5. [Health Checks](#5-health-checks)
+6. [Security Gate — docker scout CVE Scan (D-10)](#6-security-gate--docker-scout-cve-scan-d-10)
+7. [On-Call Escalation](#7-on-call-escalation)
+8. [Common Failure Modes](#8-common-failure-modes)
+9. [Phase 6 Deferred Items](#9-phase-6-deferred-items)
 
 ---
 
@@ -146,11 +147,18 @@ docker compose logs -f --tail 50
 Services start in dependency order:
 
 ```
-postgres / minio / redis  →  backend / celery-worker / celery-beat
+postgres / minio / redis  →  migrate (alembic upgrade head)
+                          →  backend / celery-worker / celery-beat
                           →  loki  →  promtail / grafana
                           →  frontend
 ```
 
+The `migrate` service is a one-shot container that runs `alembic upgrade head` using
+`DATABASE_MIGRATE_URL` (DDL-capable `docuvault_migrate` role) and exits 0 on success.
+All application services (`backend`, `celery-worker`, `celery-beat`) depend on it with
+`condition: service_completed_successfully`. If `migrate` exits non-zero, no application
+process starts and the deployment is blocked until the migration issue is resolved.
+
 Access points after a healthy startup:
 
 | Service | URL |
@@ -194,7 +202,88 @@ Do not add `read_only:` to the celery-beat block — it will exit immediately wi
 
 ---
 
-## 3. Backup Strategy
+## 3. Database Migrations
+
+### Checking the current revision
+
+```bash
+# Inside the running backend container
+docker compose exec backend alembic current
+
+# Via a one-off migrate container (preferred — uses DDL role)
+docker compose run --rm migrate alembic current
+```
+
+A healthy deployment at Phase 12 head reports:
+
+```
+0006 (head)
+```
+
+### Upgrading to the latest migration
+
+```bash
+# Compose will run migrate automatically on docker compose up.
+# To run it manually (e.g., after a git pull):
+docker compose run --rm migrate
+```
+
+`migrate` runs `alembic upgrade head` using `DATABASE_MIGRATE_URL`. On success it exits 0.
+Application services will not start until this exits 0 — this is the deployment gate.
+
+### Viewing migration history
+
+```bash
+docker compose run --rm migrate alembic history --verbose
+```
+
+### Recovering from a failed migration
+
+1. **Read the logs** — the migrate container logs contain the SQL and the error:
+   ```bash
+   docker compose logs migrate
+   ```
+
+2. **Fix the migration** — edit the failing migration revision file in `backend/migrations/versions/`.
+
+3. **Re-run** — the migrate container is idempotent; Alembic skips already-applied revisions:
+   ```bash
+   docker compose run --rm migrate
+   ```
+
+4. **Check the revision** — confirm head is applied before restarting backend:
+   ```bash
+   docker compose run --rm migrate alembic current
+   ```
+
+5. **Restart application services** after a successful migration:
+   ```bash
+   docker compose up -d backend celery-worker celery-beat
+   ```
+
+### Emergency: schema drift on live database (Phase 12 gap-closure scenario)
+
+If the backend is already running against a database that is behind head (e.g., it started
+before the migrate service was introduced and reports `UndefinedColumn: display_name_override`):
+
+```bash
+# 1. Stop application processes (not postgres)
+docker compose stop backend celery-worker celery-beat
+
+# 2. Apply missing migrations via the dedicated migrate service
+docker compose run --rm migrate
+
+# 3. Verify the revision
+docker compose run --rm migrate alembic current
+# Expected: 0006 (head)
+
+# 4. Restart application processes through the gated path
+docker compose up -d backend celery-worker celery-beat
+```
+
+---
+
+## 4. Backup Strategy
 
 ### PostgreSQL