fix(gate): clear the xenon complexity failure + fixable test warnings

- transcript_retention.py: split select_prunable_transcripts into small helpers
  so the module averages complexity rank A (was B — failed make quality / xenon).
- pyproject: move the markers table from [tool.coverage.run] (coverage warned
  'Unrecognized option') to [tool.pytest.ini_options] where it belongs.
- HTTP_422_UNPROCESSABLE_ENTITY -> HTTP_422_UNPROCESSABLE_CONTENT (old name
  deprecated) in the tasks/product/settings routes + the validation middleware.
- conftest: drop pool_pre_ping on the per-test engine — pointless for a fresh
  per-test engine and it leaves an un-awaited asyncpg Connection._cancel
  coroutine that surfaced as a RuntimeWarning across ~30 integration tests.

pyproject.toml

@@ -237,6 +237,12 @@ testpaths = ["tests"]
 python_files = ["test_*.py"]
 asyncio_default_fixture_loop_scope = "function"
 addopts = "--cov=roboco --cov-report=term-missing"
+markers = [
+    "asyncio: mark tests as async",
+    "slow: marks tests as slow",
+    "integration: marks tests as integration tests",
+    "unit: marks tests as unit tests",
+]
 
 [tool.coverage.run]
 # Coverage core. Python 3.12+ supports sys.monitoring; the legacy `pytrace`
@@ -286,12 +292,6 @@ omit = [
     # Auto-generated migrations.
     "alembic/*",
 ]
-markers = [
-    "asyncio: mark tests as async",
-    "slow: marks tests as slow",
-    "integration: marks tests as integration tests",
-    "unit: marks tests as unit tests",
-]
 
 # =============================================================================
 # Vulture Configuration