fix: disable Analyse on active recordings; guard server-side too

Clicking Analyse on a file currently being recorded caused libsndfile to
fail with 'Internal psf_fseek() failed' because an in-progress FLAC file
has no seektable (written only on close()).

Client: Analyse button is disabled with a tooltip while isRec is true.
Server: _api_analyze checks status.json and returns 409 with a readable
message if the file is still being recorded, before attempting to open it.

web.py

@@ -272,6 +272,15 @@ class _Handler(BaseHTTPRequestHandler):
         if path is None:
             return
 
+        status_path = Path(self.recordings_dir) / 'status.json'
+        try:
+            with open(status_path) as fh:
+                if filename in set(json.load(fh).get('active', [])):
+                    self._json_err(409, 'File is currently being recorded — analysis unavailable until recording stops')
+                    return
+        except Exception:
+            pass
+
         ext = path.suffix.lower()
         if ext == '.wav':
             result = analyze_wav(path, threshold=self.threshold)
@@ -691,7 +700,12 @@ async function load() {
       const abtn = document.createElement('button');
       abtn.textContent = 'Analyse';
       abtn.setAttribute('aria-label', `Analyse loudness of ${f.name}`);
+      if (isRec) {
+        abtn.disabled = true;
+        abtn.title = 'Recording in progress — analyse after recording stops';
+      } else {
         abtn.addEventListener('click', () => analyse(f.name, cell, abtn));
+      }
       cell.appendChild(abtn);
     }