fix: adopt the #392 dependency file as the single source (#42)

Rebase resolution: dev's #392 introduced tests_backend/requirements.txt
(python 3.14, phcc 0.13.357, homeassistant 2026.8.3) — exactly the
single-source-of-pins AC of this issue, so the duplicate
requirements_test.txt is dropped and both workflows keep installing from
the #392 file; ruff is added there for the lint step. The backend
reuse key no longer names the dead file (tests_backend/ as a root
already covers the new one); ARCHITECTURE.md points at the real path.

User-Visible: no
Issue: #42

docs/ARCHITECTURE.md

@@ -1549,8 +1549,10 @@ their passports; `scripts/config-audit.mjs` treats both as `current`.
 
 ## Backend quality gates (#42, 2026-08-30)
 
-- `requirements_test.txt` is the single source of backend CI dependencies
-  (validate.yml and mutation-gate.yml install from it).
+- `tests_backend/requirements.txt` is the single source of backend CI
+  dependencies (validate.yml and mutation-gate.yml install from it; the file
+  itself was introduced by #392, which also moved the harness to python 3.14
+  and the current Home Assistant — #42 adds ruff to it for the lint step).
 - `pyproject.toml` configures ruff (E/F/B/I, E501 excluded by decision) and
   mypy strict for a grow-only allowlist of pure modules; the completeness
   guard lives in `tests_backend/test_backend_quality.py`.

scripts/gate-reuse.mjs

@@ -49,13 +49,13 @@ export const HARNESS = {
       || /^demo\/benchmark_(glow|large_house)\.mjs$/.test(rel),
   },
   backend: {
-    // #42: порог покрытия и пины зависимостей — прямые входы job
-    // (`head -1 baseline` в шаге сравнения; `pip install -r`): их изменение
-    // без правок тестов обязано сбрасывать реюз, иначе baseline-bump молча
-    // пройдёт по старому зелёному маркеру.
+    // #42: порог покрытия и конфиг линтеров — прямые входы job
+    // (`head -1 baseline` в шаге сравнения; ruff/mypy читают pyproject): их
+    // изменение без правок тестов обязано сбрасывать реюз, иначе baseline-bump
+    // молча пройдёт по старому зелёному маркеру. Пины зависимостей
+    // (tests_backend/requirements.txt, #392) покрыты корнем tests_backend.
     roots: ['tests_backend', 'custom_components', 'pytest.ini',
-      'scripts/backend-coverage-baseline.txt', 'requirements_test.txt',
-      'pyproject.toml'],
+      'scripts/backend-coverage-baseline.txt', 'pyproject.toml'],
     // Внутри custom_components/** значим только Python: собранный фронтенд
     // лежит там же и меняется от любой сборки, а backend его не исполняет.
     keep: (rel) => !rel.startsWith('custom_components/') || rel.endsWith('.py'),