feat: cache analysis results alongside audio files

After the first analysis of a WAV/FLAC file, the result is written to
<filename>.analysis.json next to the audio. Subsequent requests with the
same threshold and min_gap parameters return the cached result immediately
without re-reading the audio data. The cache is invalidated automatically
if either parameter changes. Written via temp-then-replace for thread safety.

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

web.py

@@ -331,6 +331,15 @@ class _Handler(BaseHTTPRequestHandler):
         except Exception:
             pass
 
+        cache_path = path.parent / (path.name + '.analysis.json')
+        try:
+            cached = json.loads(cache_path.read_text('utf-8'))
+            if cached.get('threshold') == threshold and cached.get('min_gap') == min_gap:
+                self._send(200, json.dumps(cached['result']).encode('utf-8'), 'application/json')
+                return
+        except Exception:
+            pass
+
         ext = path.suffix.lower()
         if ext == '.wav':
             result = analyze_wav(path, threshold=threshold, min_gap=min_gap)
@@ -343,6 +352,13 @@ class _Handler(BaseHTTPRequestHandler):
             self._json_err(400, f'Loudness analysis is not available for {ext} files')
             return
 
+        try:
+            tmp = cache_path.with_suffix('.tmp')
+            tmp.write_text(json.dumps({'threshold': threshold, 'min_gap': min_gap, 'result': result}), 'utf-8')
+            os.replace(tmp, cache_path)
+        except Exception:
+            pass
+
         self._send(200, json.dumps(result).encode('utf-8'), 'application/json')
 
     def _api_status(self):