feat(notification): use event worker pool (#8962)

# Which Problems Are Solved

The current handling of notification follows the same pattern as all
other projections:
Created events are handled sequentially (based on "position") by a
handler. During the process, a lot of information is aggregated (user,
texts, templates, ...).
This leads to back pressure on the projection since the handling of
events might take longer than the time before a new event (to be
handled) is created.

# How the Problems Are Solved

- The current user notification handler creates separate notification
events based on the user / session events.
- These events contain all the present and required information
including the userID.
- These notification events get processed by notification workers, which
gather the necessary information (recipient address, texts, templates)
to send out these notifications.
- If a notification fails, a retry event is created based on the current
notification request including the current state of the user (this
prevents race conditions, where a user is changed in the meantime and
the notification already gets the new state).
- The retry event will be handled after a backoff delay. This delay
increases with every attempt.
- If the configured amount of attempts is reached or the message expired
(based on config), a cancel event is created, letting the workers know,
the notification must no longer be handled.
- In case of successful send, a sent event is created for the
notification aggregate and the existing "sent" events for the user /
session object is stored.
- The following is added to the defaults.yaml to allow configuration of
the notification workers:
```yaml

Notifications:
  # The amount of workers processing the notification request events.
  # If set to 0, no notification request events will be handled. This can be useful when running in
  # multi binary / pod setup and allowing only certain executables to process the events.
  Workers: 1 # ZITADEL_NOTIFIACATIONS_WORKERS
  # The amount of events a single worker will process in a run.
  BulkLimit: 10 # ZITADEL_NOTIFIACATIONS_BULKLIMIT
  # Time interval between scheduled notifications for request events
  RequeueEvery: 2s # ZITADEL_NOTIFIACATIONS_REQUEUEEVERY
  # The amount of workers processing the notification retry events.
  # If set to 0, no notification retry events will be handled. This can be useful when running in
  # multi binary / pod setup and allowing only certain executables to process the events.
  RetryWorkers: 1 # ZITADEL_NOTIFIACATIONS_RETRYWORKERS
  # Time interval between scheduled notifications for retry events
  RetryRequeueEvery: 2s # ZITADEL_NOTIFIACATIONS_RETRYREQUEUEEVERY
  # Only instances are projected, for which at least a projection-relevant event exists within the timeframe
  # from HandleActiveInstances duration in the past until the projection's current time
  # If set to 0 (default), every instance is always considered active
  HandleActiveInstances: 0s # ZITADEL_NOTIFIACATIONS_HANDLEACTIVEINSTANCES
  # The maximum duration a transaction remains open
  # before it spots left folding additional events
  # and updates the table.
  TransactionDuration: 1m # ZITADEL_NOTIFIACATIONS_TRANSACTIONDURATION
  # Automatically cancel the notification after the amount of failed attempts
  MaxAttempts: 3 # ZITADEL_NOTIFIACATIONS_MAXATTEMPTS
  # Automatically cancel the notification if it cannot be handled within a specific time
  MaxTtl: 5m  # ZITADEL_NOTIFIACATIONS_MAXTTL
  # Failed attempts are retried after a confogired delay (with exponential backoff).
  # Set a minimum and maximum delay and a factor for the backoff
  MinRetryDelay: 1s  # ZITADEL_NOTIFIACATIONS_MINRETRYDELAY
  MaxRetryDelay: 20s # ZITADEL_NOTIFIACATIONS_MAXRETRYDELAY
  # Any factor below 1 will be set to 1
  RetryDelayFactor: 1.5 # ZITADEL_NOTIFIACATIONS_RETRYDELAYFACTOR
```


# Additional Changes

None

# Additional Context

- closes #8931

internal/notification/channels/error.go

@@ -0,0 +1,25 @@
+package channels
+
+import "errors"
+
+type CancelError struct {
+	Err error
+}
+
+func (e *CancelError) Error() string {
+	return e.Err.Error()
+}
+
+func NewCancelError(err error) error {
+	return &CancelError{
+		Err: err,
+	}
+}
+
+func (e *CancelError) Is(target error) bool {
+	return errors.As(target, &e)
+}
+
+func (e *CancelError) Unwrap() error {
+	return e.Err
+}

internal/notification/channels/twilio/channel.go

@@ -1,7 +1,10 @@
 package twilio
 
 import (
-	newTwilio "github.com/twilio/twilio-go"
+	"errors"
+
+	"github.com/twilio/twilio-go"
+	twilioClient "github.com/twilio/twilio-go/client"
 	openapi "github.com/twilio/twilio-go/rest/api/v2010"
 	verify "github.com/twilio/twilio-go/rest/verify/v2"
 	"github.com/zitadel/logging"
@@ -12,7 +15,7 @@ import (
 )
 
 func InitChannel(config Config) channels.NotificationChannel {
-	client := newTwilio.NewRestClientWithParams(newTwilio.ClientParams{Username: config.SID, Password: config.Token})
+	client := twilio.NewRestClientWithParams(twilio.ClientParams{Username: config.SID, Password: config.Token})
 	logging.Debug("successfully initialized twilio sms channel")
 
 	return channels.HandleMessageFunc(func(message channels.Message) error {
@@ -26,6 +29,17 @@ func InitChannel(config Config) channels.NotificationChannel {
 			params.SetChannel("sms")
 
 			resp, err := client.VerifyV2.CreateVerification(config.VerifyServiceSID, params)
+
+			var twilioErr *twilioClient.TwilioRestError
+			if errors.As(err, &twilioErr) && twilioErr.Code == 60203 {
+				// If there were too many attempts to send a verification code (more than 5 times)
+				// without a verification check, even retries with backoff might not solve the problem.
+				// Instead, let the user initiate the verification again (e.g. using "resend code")
+				// https://www.twilio.com/docs/api/errors/60203
+				logging.WithFields("error", twilioErr.Message, "code", twilioErr.Code).Warn("twilio create verification error")
+				return channels.NewCancelError(twilioErr)
+			}
+
 			if err != nil {
 				return zerrors.ThrowInternal(err, "TWILI-0s9f2", "could not send verification")
 			}