fix(notifications): bring back legacy notification handling (#9015)

# Which Problems Are Solved

There are some problems related to the use of CockroachDB with the new
notification handling (#8931).
See #9002 for details.

# How the Problems Are Solved

- Brought back the previous notification handler as legacy mode.
- Added a configuration to choose between legacy mode and new parallel
workers.
  - Enabled legacy mode by default to prevent issues.

# Additional Changes

None

# Additional Context

- closes https://github.com/zitadel/zitadel/issues/9002
- relates to #8931

internal/api/ui/login/mail_verify_handler.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"net/http"
+	"net/url"
 	"slices"
 
 	"github.com/zitadel/logging"
@@ -43,6 +44,15 @@ type mailVerificationData struct {
 	HasSymbol    string
 }
 
+func MailVerificationLink(origin, userID, code, orgID, authRequestID string) string {
+	v := url.Values{}
+	v.Set(queryUserID, userID)
+	v.Set(queryCode, code)
+	v.Set(queryOrgID, orgID)
+	v.Set(QueryAuthRequestID, authRequestID)
+	return externalLink(origin) + EndpointMailVerification + "?" + v.Encode()
+}
+
 func MailVerificationLinkTemplate(origin, userID, orgID, authRequestID string) string {
 	return fmt.Sprintf("%s%s?%s=%s&%s=%s&%s=%s&%s=%s",
 		externalLink(origin), EndpointMailVerification,