docs(05): add UAT, UI-SPEC, deferred items, debug notes; refine plans 09-11

Plan refinements: Vitest tests added to 09/10 must-haves, explicit
mock_flow two-tuple pattern in 10, test_admin_api.py fixture usage in 11.
New artifacts: UAT checklist, UI-SPEC, deferred-items, debug investigation
for cloud-doc-operations-fail.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.planning/phases/05-cloud-storage-backends/deferred-items.md

@@ -0,0 +1,15 @@
+# Phase 5 — Deferred Items
+
+## Pre-existing issues discovered during Plan 05-01 execution (out of scope)
+
+### test_extractor.py::test_extract_docx — ModuleNotFoundError: No module named 'docx'
+
+- **Discovered during:** Task 3 (full pytest run)
+- **Root cause:** `python-docx` package is not installed in the local Python environment.
+  The backend runs in Docker where all requirements.txt packages are installed, but the
+  local test runner uses the system Python 3.9.6 which does not have a virtualenv
+  with all requirements installed.
+- **Not caused by:** Plan 05-01 changes (requirements.txt, config.py, conftest.py, test_cloud.py)
+- **Resolution path:** Install python-docx in the local test environment:
+  `pip3 install python-docx` or run tests inside Docker with all deps available.
+- **Impact:** 1 pre-existing failure in test_extractor.py; does not affect Phase 5 work.