fix: cinematic render is_cinematic detection and usd_path str/Path handling

Three bugs causing all cinematic renders to silently fall through to the
turntable path and then immediately crash:

1. is_cinematic read from filtered invocation-override dict — 'cinematic' key
   is not in the allowed override key list for turntable_video, so it gets
   stripped. Fixed: read directly from output_type.render_settings.

2. render_turntable_to_file and render_cinematic_to_file both call
   usd_path.exists() but receive a str from the caller. Added the same
   isinstance str→Path conversion that render_still_to_file already had.

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

backend/app/services/render_blender.py

@@ -566,6 +566,9 @@ def render_turntable_to_file(
 
     t0 = time.monotonic()
 
+    if isinstance(usd_path, str) and usd_path.strip():
+        usd_path = Path(usd_path)
+
     # 1. GLB conversion (OCC) — skipped when usd_path is provided
     use_usd = bool(usd_path and usd_path.exists())
 
@@ -804,6 +807,9 @@ def render_cinematic_to_file(
 
     t0 = time.monotonic()
 
+    if isinstance(usd_path, str) and usd_path.strip():
+        usd_path = Path(usd_path)
+
     # 1. GLB conversion (OCC) — skipped when usd_path is provided
     use_usd = bool(usd_path and usd_path.exists())