fix(cli): validate ffmpeg availability at startup

Move ffmpeg check from FFMPEGVideoWriter to argument validation phase.
Prevents wasted GPU processing time when ffmpeg backend is selected
but ffmpeg is not installed.

inference_cli.py

@@ -164,9 +164,6 @@ class FFMPEGVideoWriter:
     """
     
     def __init__(self, path: str, width: int, height: int, fps: float, use_10bit: bool = False):
-        if shutil.which("ffmpeg") is None:
-            raise RuntimeError("ffmpeg not found in PATH. Install ffmpeg or use --video_backend opencv")
-        
         pix_fmt = 'yuv420p10le' if use_10bit else 'yuv420p'
         codec = 'libx265' if use_10bit else 'libx264'
         
@@ -1517,6 +1514,12 @@ def main() -> None:
         debug.log(f"VAE decode tile overlap ({args.vae_decode_tile_overlap}) must be smaller than tile size ({args.vae_decode_tile_size})", level="ERROR", category="vae", force=True)
         sys.exit(1)
     
+    # Validate ffmpeg availability if selected
+    if args.video_backend == "ffmpeg" and shutil.which("ffmpeg") is None:
+        debug.log("--video_backend ffmpeg requires ffmpeg in PATH. Install ffmpeg or use --video_backend opencv", 
+                 level="ERROR", category="setup", force=True)
+        sys.exit(1)
+    
     # Inform about caching defaults
     if args.cache_dit and args.dit_offload_device == "none":
         offload_target = "system memory (CPU)" if get_gpu_backend() != "mps" else "unified memory"