Fix JWT validation by configuring custom signing key resolver

- Added IssuerSigningKeyResolver to fetch JWKS directly from internal Keycloak URL
- This bypasses the localhost:8080 URLs in Keycloak's discovery document
- Ensures JWT tokens are validated against correct signing keys

backend/WorkClub.Api/Program.cs

@@ -51,21 +51,53 @@ builder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
         options.RequireHttpsMetadata = false;
         options.MapInboundClaims = false;
         
-        // For Docker internal communication, use the direct Keycloak URL for metadata
+        // For Docker internal communication, configure metadata and signing key resolution
-        // This bypasses the hostname mismatch in Keycloak's discovery endpoint
+        // to bypass the hostname mismatch in Keycloak's discovery endpoint
         var keycloakAuthority = builder.Configuration["Keycloak:Authority"];
+        var keycloakInternalUrl = "http://keycloak:8081";
+        
         if (keycloakAuthority?.Contains("keycloak:") == true)
         {
+            // Set metadata address to internal Keycloak URL
             options.MetadataAddress = $"{keycloakAuthority}/.well-known/openid-configuration";
-        }
             
+            // Configure custom signing key resolver to fetch from internal Keycloak URL
+            // This overrides the URLs returned in the discovery document
+            var httpClient = new HttpClient();
             options.TokenValidationParameters = new Microsoft.IdentityModel.Tokens.TokenValidationParameters
             {
-            ValidateIssuer = false, // Disabled for local dev - external clients use localhost:8080, internal use keycloak:8080
+                ValidateIssuer = false,
+                ValidateAudience = true,
+                ValidateLifetime = true,
+                ValidateIssuerSigningKey = true,
+                IssuerSigningKeyResolver = (token, securityToken, kid, validationParameters) =>
+                {
+                    // Fetch JWKS from internal Keycloak URL
+                    var jwksUrl = $"{keycloakInternalUrl}/realms/workclub/protocol/openid-connect/certs";
+                    try
+                    {
+                        var response = httpClient.GetStringAsync(jwksUrl).GetAwaiter().GetResult();
+                        var jwks = new Microsoft.IdentityModel.Tokens.JsonWebKeySet(response);
+                        return jwks.Keys;
+                    }
+                    catch (Exception ex)
+                    {
+                        Console.WriteLine($"Failed to fetch JWKS from {jwksUrl}: {ex.Message}");
+                        return Array.Empty<Microsoft.IdentityModel.Tokens.SecurityKey>();
+                    }
+                }
+            };
+        }
+        else
+        {
+            options.TokenValidationParameters = new Microsoft.IdentityModel.Tokens.TokenValidationParameters
+            {
+                ValidateIssuer = false,
                 ValidateAudience = true,
                 ValidateLifetime = true,
                 ValidateIssuerSigningKey = true
             };
+        }
         options.Events = new JwtBearerEvents
         {
             OnAuthenticationFailed = context =>