List scene templates, optionally filtered by category.

<br/>Scene templates are global presets (nullable owner_id per<br/>CLAUDE.md §3 — "everyone reads; only admins write"), so this GET<br/>is intentionally public-read. The auth gate in create_app()<br/>still gates the path in production where Cognito is configured;<br/>in test / CodeBuild safety-check environments without Cognito the<br/>gate skips and this list is reachable anonymously, which is what<br/>tests/test_safety.py::test_api_returns_200 asserts.<br/><br/>The @login_required originally landed here in 952c915<br/>("full route hardening") but contradicts both the global-preset<br/>design and the safety smoke test, so it was removed.