feat(02-03): add waveform extraction module and HTTP endpoint

- Create lightsync/audio/waveform.py with extract_peaks() (AUD-05)
- MP3 via ffmpeg pipe, WAV/FLAC/OGG via soundfile+numpy
- Add GET /api/audio/waveform endpoint with asyncio.to_thread
- peaks param clamped 100-5000, returns {peaks, count, file}

lightsync/audio/waveform.py

@@ -0,0 +1,58 @@
+"""Waveform peak extraction for timeline display (AUD-05)."""
+import subprocess
+from pathlib import Path
+
+import numpy as np
+import soundfile as sf
+
+
+def extract_peaks(path: str, num_peaks: int = 1000) -> list[float]:
+    """Extract downsampled peak amplitudes from audio file.
+
+    Returns a list of floats (0.0-1.0) representing peak amplitude per chunk.
+    Handles MP3 via ffmpeg subprocess (libsndfile doesn't support MP3).
+
+    Args:
+        path: Absolute path to audio file
+        num_peaks: Number of peak samples to return (default 1000)
+
+    Returns:
+        List of float peak values, length <= num_peaks
+    """
+    p = Path(path)
+    if p.suffix.lower() == ".mp3":
+        return _extract_peaks_mp3(path, num_peaks)
+    return _extract_peaks_soundfile(path, num_peaks)
+
+
+def _extract_peaks_soundfile(path: str, num_peaks: int) -> list[float]:
+    """Extract peaks using soundfile (WAV, FLAC, OGG)."""
+    data, _ = sf.read(path, always_2d=True)
+    mono = np.mean(data, axis=1)
+    return _downsample_peaks(mono, num_peaks)
+
+
+def _extract_peaks_mp3(path: str, num_peaks: int) -> list[float]:
+    """Extract peaks from MP3 via ffmpeg pipe to float32 PCM."""
+    cmd = [
+        "ffmpeg", "-i", path,
+        "-f", "f32le", "-ar", "44100", "-ac", "1",
+        "pipe:1", "-loglevel", "quiet",
+    ]
+    result = subprocess.run(cmd, capture_output=True, timeout=60)
+    if result.returncode != 0:
+        raise RuntimeError(f"ffmpeg failed for {path}: exit code {result.returncode}")
+    mono = np.frombuffer(result.stdout, dtype=np.float32)
+    return _downsample_peaks(mono, num_peaks)
+
+
+def _downsample_peaks(mono: np.ndarray, num_peaks: int) -> list[float]:
+    """Downsample mono audio to peak amplitude array."""
+    if len(mono) == 0:
+        return []
+    chunk_size = max(1, len(mono) // num_peaks)
+    peaks = []
+    for i in range(0, len(mono), chunk_size):
+        chunk = mono[i : i + chunk_size]
+        peaks.append(float(np.max(np.abs(chunk))))
+    return peaks[:num_peaks]