feat: cache waveform data to disk, skip ffmpeg on reload

Waveform peaks are saved as .npy files keyed by MD5 of the video
path. Subsequent loads of the same video read from cache instead
of re-running ffmpeg extraction.

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

main.py

@@ -1368,8 +1368,14 @@ class ScanResultsPanel(QWidget):
             super().keyPressEvent(event)
 
 
+_WAVEFORM_CACHE_DIR = os.path.join(
+    os.path.dirname(os.path.dirname(os.path.abspath(__file__))),
+    "cache", "waveforms",
+)
+
+
 class WaveformWorker(QThread):
-    """Extract a low-res waveform envelope in the background."""
+    """Extract a low-res waveform envelope in the background (with disk cache)."""
     done = pyqtSignal(object)  # emits numpy array of peak values
 
     def __init__(self, video_path: str, n_bins: int = 2000):
@@ -1377,9 +1383,22 @@ class WaveformWorker(QThread):
         self._path = video_path
         self._n_bins = n_bins
 
+    @staticmethod
+    def _cache_path(video_path: str) -> str:
+        import hashlib
+        h = hashlib.md5(video_path.encode()).hexdigest()
+        return os.path.join(_WAVEFORM_CACHE_DIR, f"{h}.npy")
+
     def run(self):
         import numpy as np
         try:
+            # Check cache first
+            cache = self._cache_path(self._path)
+            if os.path.exists(cache):
+                peaks = np.load(cache)
+                self.done.emit(peaks)
+                return
+
             cmd = [
                 _bin("ffmpeg"), "-i", self._path,
                 "-vn", "-ac", "1", "-ar", "8000",
@@ -1399,6 +1418,9 @@ class WaveformWorker(QThread):
             mx = peaks.max()
             if mx > 0:
                 peaks = peaks / mx
+            # Save to cache
+            os.makedirs(_WAVEFORM_CACHE_DIR, exist_ok=True)
+            np.save(cache, peaks)
             self.done.emit(peaks)
         except Exception:
             pass