test: improve test isolation and setup management

Add Token and RegularToken fields to TestServerSetup for pre-authenticated
access. Update setupTestServer to create fresh users with valid tokens at
initialization time. Simplify createTestMediaItemID to use setup.Token.
Remove loginTestUser, loginRegularUser, loginAdminUser functions in favor
of setup.Token/setup.RegularToken. Update createTestUserOnce and
getTestUserID/getRegularUserID to be idempotent. Update all test files to
use setup.Token instead of calling login helpers.

cmd/server/tests/websocket_test.go

@@ -20,7 +20,7 @@ import (
 setup := setupTestServer(t)
 
 // Get JWT token for a test user
-token := loginTestUser(t, setup.Server, setup.DB)
+token := setup.Token
 
 // Connect to WebSocket endpoint
 wsURL := strings.Replace(setup.Server.URL, "http", "ws", 1) + "/ws/sync?token=" + token
@@ -83,7 +83,7 @@ func TestWebSocketDeviceAuth(t *testing.T) {
 setup := setupTestServer(t)
 
 // Get JWT token
-token := loginTestUser(t, setup.Server, setup.DB)
+token := setup.Token
 
 // Create a test media item
 userID := getTestUserID(t, setup.DB)
@@ -144,7 +144,7 @@ assert.Equal(t, "web", sourceDevice["type"])
 /* func TestWebSocketPingPong(t *testing.T) {
 setup := setupTestServer(t)
 
-token := loginTestUser(t, setup.Server, setup.DB)
+token := setup.Token
 
 wsURL := strings.Replace(setup.Server.URL, "http", "ws", 1) + "/ws/sync?token=" + token
 ws, _, err := websocket.DefaultDialer.Dial(wsURL, nil)
@@ -176,7 +176,7 @@ if err == nil {
 /* func TestWebSocketConnectionLimit(t *testing.T) {
 	setup := setupTestServer(t)
 
-	token := loginTestUser(t, setup.Server, setup.DB)
+	token := setup.Token
 
 	// Create multiple connections
 	connections := make([]*websocket.Conn, 5)