Reduce frontend polling load and clean stale UI hooks

Endpoints/SuggestionValidator.cs

@@ -1,8 +1,14 @@
+using System.Collections.Concurrent;
+
 namespace GameList.Endpoints;
 
 internal static class SuggestionValidator
 {
-    public static async Task<string?> ValidateAsync(SuggestionInput input, IHttpClientFactory httpFactory)
+    private static readonly ConcurrentDictionary<string, (bool Reachable, DateTimeOffset ExpiresAt)> ImageReachabilityCache = new(StringComparer.OrdinalIgnoreCase);
+    private static readonly TimeSpan ReachableCacheTtl = TimeSpan.FromMinutes(15);
+    private static readonly TimeSpan UnreachableCacheTtl = TimeSpan.FromMinutes(2);
+
+    public static async Task<string?> ValidateAsync(SuggestionInput input, IHttpClientFactory httpFactory, bool shouldValidateImageReachability = true)
     {
         if (string.IsNullOrWhiteSpace(input.Name) || input.Name.Length > 100)
             return "Name is required and must be <= 100 characters.";
@@ -10,7 +16,7 @@ internal static class SuggestionValidator
         if (!EndpointHelpers.IsValidImageUrl(input.ScreenshotUrl))
             return "Screenshot URL must be http(s) and end with an image file extension.";
 
-        if (!await EndpointHelpers.IsReachableImageAsync(input.ScreenshotUrl, httpFactory))
+        if (shouldValidateImageReachability && !await IsReachableImageCachedAsync(input.ScreenshotUrl, httpFactory))
             return "Screenshot URL could not be validated as an image. Use a public image link (http/https, no redirects, max 5 MB).";
 
         if (!EndpointHelpers.IsValidHttpUrl(input.GameUrl))
@@ -22,6 +28,21 @@ internal static class SuggestionValidator
         return ValidatePlayers(input.MinPlayers, input.MaxPlayers);
     }
 
+    private static async Task<bool> IsReachableImageCachedAsync(string? url, IHttpClientFactory httpFactory)
+    {
+        if (string.IsNullOrWhiteSpace(url))
+            return true;
+
+        var normalized = url.Trim();
+        if (ImageReachabilityCache.TryGetValue(normalized, out var cached) && cached.ExpiresAt > DateTimeOffset.UtcNow)
+            return cached.Reachable;
+
+        var reachable = await EndpointHelpers.IsReachableImageAsync(normalized, httpFactory);
+        var ttl = reachable ? ReachableCacheTtl : UnreachableCacheTtl;
+        ImageReachabilityCache[normalized] = (reachable, DateTimeOffset.UtcNow.Add(ttl));
+        return reachable;
+    }
+
     private static string? ValidatePlayers(int? minPlayers, int? maxPlayers)
     {
         if (minPlayers is null && maxPlayers is null)