refactor: cinematic render — linear keyframes, 3 segments, 250 frames, white bg

Changes per user feedback:
- Keyframe interpolation: BEZIER → LINEAR (all fcurves set to LINEAR)
- Removed segment 4 (closeup) — now 3 segments only
- Frame count: 480 → 250 (10 seconds at 25fps)
- FPS: 24 → 25
- Easing removed — pure linear interpolation between segment params
- White background by default (World node Color = white)
- Transparent bg still available as override

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/render_blender.py

@@ -563,8 +563,8 @@ def render_cinematic_to_file(
     import time
 
     # Cinematic parameters are fixed
-    frame_count = 480
-    fps = 24
+    frame_count = 250
+    fps = 25
 
     blender_bin = find_blender()
     if not blender_bin: