Fix multi-res HLS encoding on CPU: fall back to libx264 when NVENC unavailable

The hardcoded h264_nvenc encoder fails on CPU-only workers. Now uses
check_nvenc_available() to auto-detect and falls back to libx264.

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

l1/streaming/multi_res_output.py

@@ -21,6 +21,8 @@ from dataclasses import dataclass, field
 
 import numpy as np
 
+from streaming.output import check_nvenc_available
+
 # Try GPU imports
 try:
     import cupy as cp
@@ -222,14 +224,26 @@ class MultiResolutionHLSOutput:
                 "-vf", f"scale={quality.width}:{quality.height}:flags=lanczos",
             ])
 
-        # NVENC encoding with quality settings
+        # Encoding settings - use NVENC if available, fall back to libx264
+        use_nvenc = check_nvenc_available()
+        if use_nvenc:
+            cmd.extend([
+                "-c:v", "h264_nvenc",
+                "-preset", "p4",  # Balanced speed/quality
+                "-tune", "hq",
+                "-b:v", f"{quality.bitrate}k",
+                "-maxrate", f"{int(quality.bitrate * 1.5)}k",
+                "-bufsize", f"{quality.bitrate * 2}k",
+            ])
+        else:
+            cmd.extend([
+                "-c:v", "libx264",
+                "-preset", "fast",
+                "-b:v", f"{quality.bitrate}k",
+                "-maxrate", f"{int(quality.bitrate * 1.5)}k",
+                "-bufsize", f"{quality.bitrate * 2}k",
+            ])
         cmd.extend([
-            "-c:v", "h264_nvenc",
-            "-preset", "p4",  # Balanced speed/quality
-            "-tune", "hq",
-            "-b:v", f"{quality.bitrate}k",
-            "-maxrate", f"{int(quality.bitrate * 1.5)}k",
-            "-bufsize", f"{quality.bitrate * 2}k",
             "-g", str(int(self.fps * self.segment_duration)),  # Keyframe interval = segment duration
             "-keyint_min", str(int(self.fps * self.segment_duration)),
             "-sc_threshold", "0",  # Disable scene change detection for consistent segments