Fix Demucs segment size: htdemucs max is 7.8, use 7

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

transcriber/separate.py

@@ -31,7 +31,7 @@ def separate(input_path: str, output_dir: str, model: str = "htdemucs") -> dict:
         python, "-m", "demucs",
         "--name", model,
         "--out", str(output_dir),
-        "--segment", "10",  # Process in 10s chunks to limit memory usage
+        "--segment", "7",  # Process in 7s chunks (htdemucs max is 7.8)
         str(input_path),
     ]