Fix captcha rendering and refresh rate limit

src/Jiaowu.Api/Controllers/AuthController.cs

@@ -138,7 +138,7 @@ public sealed class AuthController(
     }
 
     [AllowAnonymous]
-    [EnableRateLimiting("public-auth")]
+    [EnableRateLimiting("captcha-challenge")]
     [HttpPost("login/captcha")]
     public async Task<ActionResult<LoginCaptchaResponse>> CreateLoginCaptcha(CancellationToken cancellationToken)
     {

src/Jiaowu.Api/Program.cs

@@ -598,6 +598,16 @@ builder.Services.AddRateLimiter(options =>
                 QueueLimit = 0,
                 AutoReplenishment = true
             }));
+    options.AddPolicy("captcha-challenge", context =>
+        RateLimitPartition.GetFixedWindowLimiter(
+            context.Connection.RemoteIpAddress?.ToString() ?? "unknown",
+            _ => new FixedWindowRateLimiterOptions
+            {
+                PermitLimit = 30,
+                Window = TimeSpan.FromMinutes(5),
+                QueueLimit = 0,
+                AutoReplenishment = true
+            }));
     options.AddPolicy("token-refresh", context =>
         RateLimitPartition.GetFixedWindowLimiter(
             context.Connection.RemoteIpAddress?.ToString() ?? "unknown",