stemserve: refresh index in a background thread

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

server/stemserve.py

@@ -93,17 +93,23 @@ class Index:
         self._by_key: dict[str, dict] = {}
 
     def get(self):
-        if time.monotonic() - self._at > INDEX_TTL:
-            self.refresh()
         with self._lock:
             return self._body, self._etag
 
     def entry(self, key):
-        if time.monotonic() - self._at > INDEX_TTL:
-            self.refresh()
         with self._lock:
             return self._by_key.get(key)
 
+    def start_background(self):
+        def loop():
+            while True:
+                time.sleep(INDEX_TTL)
+                try:
+                    self.refresh()
+                except Exception as e:
+                    log(f"index refresh failed: {e}")
+        threading.Thread(target=loop, daemon=True).start()
+
     @property
     def count(self):
         return self._count
@@ -537,6 +543,7 @@ def main():
         sys.exit(f"STEMS_DIR not found: {STEMS_DIR}")
     os.makedirs(CACHE_DIR, exist_ok=True)
     INDEX.refresh()
+    INDEX.start_background()
     log(f"stemserve on :{PORT}, {INDEX.count} tracks in {STEMS_DIR}, ffmpeg={'yes' if FFMPEG else 'no'}, "
         f"token={'on' if TOKEN else 'off'}")
     server = QuietServer(("0.0.0.0", PORT), Handler)