Fix lazy audio path resolution for GPU streaming

Audio playback path was being resolved during parsing when database
may not be ready, causing fallback to non-existent path. Now resolves
lazily when stream starts, matching how audio analyzer works.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

sexp_effects/primitive_libs/streaming.py

@@ -65,11 +65,20 @@ class VideoSource:
         self._last_read_time = -1
         self._cached_frame = None
 
+        # Check if file exists
+        if not self.path.exists():
+            raise FileNotFoundError(f"Video file not found: {self.path}")
+
         # Get video info
         cmd = ["ffprobe", "-v", "quiet", "-print_format", "json",
                "-show_streams", str(self.path)]
         result = subprocess.run(cmd, capture_output=True, text=True)
-        info = json.loads(result.stdout)
+        if result.returncode != 0:
+            raise RuntimeError(f"Failed to probe video '{self.path}': {result.stderr}")
+        try:
+            info = json.loads(result.stdout)
+        except json.JSONDecodeError:
+            raise RuntimeError(f"Invalid video file or ffprobe failed: {self.path}")
 
         for stream in info.get("streams", []):
             if stream.get("codec_type") == "video":
@@ -281,16 +290,27 @@ class AudioAnalyzer:
         self.path = Path(path)
         self.sample_rate = sample_rate
 
+        # Check if file exists
+        if not self.path.exists():
+            raise FileNotFoundError(f"Audio file not found: {self.path}")
+
         # Load audio via ffmpeg
-        cmd = ["ffmpeg", "-v", "quiet", "-i", str(self.path),
+        cmd = ["ffmpeg", "-v", "error", "-i", str(self.path),
                "-f", "f32le", "-ac", "1", "-ar", str(sample_rate), "-"]
         result = subprocess.run(cmd, capture_output=True)
+        if result.returncode != 0:
+            raise RuntimeError(f"Failed to load audio '{self.path}': {result.stderr.decode()}")
         self._audio = np.frombuffer(result.stdout, dtype=np.float32)
+        if len(self._audio) == 0:
+            raise RuntimeError(f"Audio file is empty or invalid: {self.path}")
 
         # Get duration
         cmd = ["ffprobe", "-v", "quiet", "-print_format", "json",
                "-show_format", str(self.path)]
-        info = json.loads(subprocess.run(cmd, capture_output=True, text=True).stdout)
+        result = subprocess.run(cmd, capture_output=True, text=True)
+        if result.returncode != 0:
+            raise RuntimeError(f"Failed to probe audio '{self.path}': {result.stderr}")
+        info = json.loads(result.stdout)
         self.duration = float(info.get("format", {}).get("duration", 60))
 
         # Beat detection state