feat: mark missing files as pending and auto-mount series volume when Sonarr is active

Add "pending" flag to cached() error response for missing files to enable download status detection. Update docker-compose to always mount series volume with fallback default path instead of requiring manual uncomment. Update README to clarify that SERIES_MEDIA_VOLUME must point to actual host path when Sonarr is enabled.

app.py

@@ -124,7 +124,7 @@ def run_ffprobe(path):
 
 def cached(path):
     p = Path(path)
-    if not p.exists(): return {"error": f"Datei nicht gefunden: {p}"}
+    if not p.exists(): return {"error": f"Datei nicht gefunden: {p}", "pending": True}
     mtime = p.stat().st_mtime; con = db(); row = con.execute("SELECT mtime,data FROM media_cache WHERE path=?", (str(p),)).fetchone(); con.close()
     if row and float(row["mtime"]) == float(mtime): return json.loads(row["data"])
     return None