api: Cache-Control no-store on /api/history (mutable JSON must not be cached)

server/app.py

@@ -136,7 +136,11 @@ def _save_hist(user: str, entries):
 
 @app.get("/api/history")
 def api_history(request: Request):
-    return sorted(_load_hist(request.state.user), key=lambda e: e.get("t", 0), reverse=True)[:5]
+    # no-store: this JSON mutates while a tab is open (position saves);
+    # a stale copy has caused "poster gone" confusion before
+    return JSONResponse(
+        sorted(_load_hist(request.state.user), key=lambda e: e.get("t", 0), reverse=True)[:5],
+        headers={"Cache-Control": "no-store"})
 
 
 @app.post("/api/history")