Add support for a time placeholder in URLs

PluralKit.Bot/Utils/AvatarUtils.cs

@@ -14,6 +14,10 @@ public static class AvatarUtils
 
     private static readonly string DiscordMediaUrlReplacement =
         "https://media.discordapp.net/attachments/$1/$2/$3.$4?width=256&height=256";
+        
+    // Rewrite time "cachebuster" parameters for randomly generated/chosen avatars with custom URLs.
+    // Number match uses `[1-9][0-9]{0,6}` rather than `[0-9]+` to avoid needing to deal with special cases for zero and limit to reasonable numbers.
+    private static readonly Regex TimePlaceholder = new(@"\{time(?:/(?<divisor>[1-9][0-9]{0,6}))?(?:%(?<modulus>[1-9][0-9]{0,6}))?\}", RegexOptions.IgnoreCase);
 
     public static string? TryRewriteCdnUrl(string? url)
     {
@@ -25,8 +29,24 @@ public static class AvatarUtils
         if (match.Groups["query"].Success)
             newUrl += "&" + match.Groups["query"].Value;
 
+        newUrl = TimePlaceholder.Replace(newUrl, ProcessTimePlaceholder);
+
         return newUrl;
     }
+    
+    private static string? ProcessTimePlaceholder(Match m) {
+        // Minutes are the maximum accuracy to avoid too much cache thrashing
+        // AND with the maximum positive value so it's always positive (as if this code will exist long enough for the 64-bit signed unix time to go negative...)
+        var time = (DateTimeOffset.UtcNow.ToUnixTimeSeconds()/60)&Int64.MaxValue;
+
+        if (m.Groups["divisor"].Success)
+            time /= Int32.Parse(m.Groups["divisor"].Value); // as above - guaranteed to not throw and be > 0
+
+        if (m.Groups["modulus"].Success)
+            time %= Int32.Parse(m.Groups["modulus"].Value);
+
+        return time.ToString();
+    }
 
     public static bool IsDiscordCdnUrl(string? url) => url != null && DiscordCdnUrl.Match(url).Success;
 }