A set of additional outbound ActivityPub outbound delivery optimizations (#4750)

* fix: remove optimistic UI updates in external actions to prevent race condition (#4711)

When adding/deleting external actions, the UI would sometimes show
duplicated or cloned rows due to a race condition between:
1. The optimistic setActions() call updating local state
2. The save() callback updating externalActions in context
3. The useEffect syncing actions from externalActions

This fix removes the optimistic updates and lets the server response
be the single source of truth, updating the UI only after the context
is updated via the save() success callback.

Fixes #4347

* fix(ap): additional outbound ActivityPub delivery optimizations

- check circuit breaker before signing any messages or any other work
- create outbound messages in batches and not all at once
- add small delay between outbound messages to spread it out a bit

* fix(ap): increase ap queue buffer size, reduce outbound http client timeout

* fix(ap): handle malformed inbox URIs

* fix(ap): batch outbox based on work performed not index

* feat(ap): add retryable activitypub outbound delivery client

---------

Co-authored-by: John Costa <jcosta@execonline.com>

activitypub/outbox/outbox.go

@@ -6,6 +6,7 @@ import (
 	"path/filepath"
 	"regexp"
 	"strings"
+	"time"
 
 	"github.com/go-fed/activity/streams"
 	"github.com/go-fed/activity/streams/vocab"
@@ -233,6 +234,7 @@ func getHashtagLinkHTMLFromTagString(baseHashtag string) string {
 }
 
 // SendToFollowers will send an arbitrary payload to all follower inboxes.
+// Requests are batched to prevent resource exhaustion when there are many followers.
 func SendToFollowers(payload []byte) error {
 	configRepository := configrepository.Get()
 	localActor := apmodels.MakeLocalIRIForAccount(configRepository.GetDefaultFederationUsername())
@@ -243,8 +245,28 @@ func SendToFollowers(payload []byte) error {
 		return errors.New("unable to fetch followers to send payload to")
 	}
 
-	for _, follower := range followers {
-		inbox, _ := url.Parse(follower.Inbox)
+	// Batch size and delay to prevent resource exhaustion during delivery.
+	// This spreads CPU load from cryptographic signing over time.
+	const batchSize = 50
+	const batchDelay = 100 * time.Millisecond
+
+	queued := 0
+	skipped := 0
+
+	for i, follower := range followers {
+		inbox, err := url.Parse(follower.Inbox)
+		if err != nil {
+			log.Errorln("unable to parse follower inbox URL", follower.Inbox, err)
+			continue
+		}
+
+		// Pre-check circuit breaker BEFORE expensive cryptographic signing.
+		// This saves CPU cycles for domains we know are failing.
+		if workerpool.ShouldSkipDomain(inbox.Host) {
+			skipped++
+			continue
+		}
+
 		req, err := crypto.CreateSignedRequest(payload, inbox, localActor)
 		if err != nil {
 			log.Errorln("unable to create outbox request", follower.Inbox, err)
@@ -252,7 +274,21 @@ func SendToFollowers(payload []byte) error {
 		}
 
 		workerpool.AddToOutboundQueue(req)
+		queued++
+
+		// Add a small delay between batches to spread out CPU and network load.
+		// This helps prevent ActivityPub delivery from competing with video encoding.
+		// Use queued count (not loop index) to ensure consistent rate limiting
+		// even when followers are skipped due to circuit breaker or parse errors.
+		if queued%batchSize == 0 && i+1 < len(followers) {
+			time.Sleep(batchDelay)
+		}
 	}
+
+	if skipped > 0 {
+		log.Debugf("Skipped %d followers due to circuit breaker, queued %d", skipped, queued)
+	}
+
 	return nil
 }