docs(12): add root causes from diagnosis

.planning/phases/12-cloud-resource-foundation/12-UAT.md

@@ -1,5 +1,5 @@
 ---
-status: partial
+status: diagnosed
 phase: 12-cloud-resource-foundation
 source:
   - 12-01-SUMMARY.md
@@ -68,16 +68,29 @@ blocked: 4
   reason: "User reported: GET /api/cloud/connections raises psycopg.errors.UndefinedColumn because cloud_connections.display_name_override does not exist in the live PostgreSQL schema."
   severity: blocker
   test: 1
-  root_cause: ""
-  artifacts: []
-  missing: []
-  debug_session: ""
+  root_cause: "The live database is at Alembic revision 0005 because docker-compose.yml starts backend/workers directly and has no migration service or dependency that runs alembic upgrade head. ORM metadata expects revision 0006."
+  artifacts:
+    - path: "docker-compose.yml"
+      issue: "No migration service; backend and Celery services depend only on PostgreSQL health."
+    - path: "backend/migrations/versions/0006_cloud_resource_foundation.py"
+      issue: "Required migration exists but is not applied during Compose startup."
+    - path: "backend/tests/test_alembic.py"
+      issue: "No PostgreSQL regression covering upgrade from revision 0005 to head."
+  missing:
+    - "Run alembic upgrade head before backend and worker startup."
+    - "Add cold-start migration regression and deployment documentation."
+  debug_session: ".planning/debug/phase-12-cloud-schema-cold-start.md"
 - truth: "A user can connect Nextcloud accounts and each connected account appears as an independently renameable cloud root."
   status: failed
   reason: "User reported: Cannot connect Nextcloud test users; POST /api/cloud/connections/webdav fails because cloud_connections.display_name_override does not exist."
   severity: blocker
   test: 2
-  root_cause: ""
-  artifacts: []
-  missing: []
-  debug_session: ""
+  root_cause: "Same schema-drift root cause as Test 1: Nextcloud connection creation queries the Phase 12 ORM while PostgreSQL remains at revision 0005."
+  artifacts:
+    - path: "docker-compose.yml"
+      issue: "No migration gate before the cloud connection API starts."
+    - path: "backend/api/cloud/connections.py"
+      issue: "Correctly queries CloudConnection, which exposes the unapplied schema immediately."
+  missing:
+    - "Apply revision 0006/head before accepting cloud connection requests."
+  debug_session: ".planning/debug/phase-12-cloud-schema-cold-start.md"