feat(webhooks): jittered exponential backoff and delivery log dashboard endpoint

Adds a 50% jitter factor to the BullMQ exponential backoff so concurrent
retries to a recovering destination don't stampede in lockstep, and
exposes the delivery history via GET /api/v1/webhooks/deliveries with
optional url/event/state filters so admins can inspect delivery logs
from the webhook dashboard.

Closes #1129

Author: lorenzo-romano

backend/src/routes/webhooks.ts

@@ -7,6 +7,7 @@ import type {
 import {
     canonicalizeWebhookPayload,
     enqueueWebhookDeliveries,
+    getDeliveryHistory,
     verifyWebhookSignature,
 } from '../services/webhooks/index.js';
 import logger from '../utils/logger.js';
@@ -55,6 +56,39 @@ router.get('/health', async (_req: Request, res: Response) => {
   });
 });
 
+// GET /deliveries — delivery log inspection for the admin dashboard.
+// Returns recent webhook delivery attempts (in-memory rolling window),
+// optionally filtered by destination URL or event type.
+router.get('/deliveries', async (req: Request, res: Response) => {
+  try {
+    const { url, event, state, limit } = req.query;
+
+    let deliveries = getDeliveryHistory();
+
+    if (typeof url === 'string' && url.length > 0) {
+      deliveries = deliveries.filter((d) => d.destinationUrl.includes(url));
+    }
+    if (typeof event === 'string' && event.length > 0) {
+      deliveries = deliveries.filter((d) => d.eventType === event);
+    }
+    if (typeof state === 'string' && state.length > 0) {
+      deliveries = deliveries.filter((d) => d.state === state);
+    }
+
+    const parsedLimit = Number(limit);
+    const capped = Number.isFinite(parsedLimit) && parsedLimit > 0 ? Math.min(parsedLimit, 500) : 100;
+
+    return res.status(200).json({
+      status: 'success',
+      count: deliveries.length,
+      deliveries: deliveries.slice(0, capped),
+    });
+  } catch (error) {
+    logger.error('Failed to list webhook deliveries:', error);
+    return res.status(500).json({ error: 'Failed to list webhook deliveries' });
+  }
+});
+
 import {
   getStellarWebhookSecret,
   processStellarHorizonWebhook,

backend/src/services/webhooks/queue.ts

@@ -5,11 +5,23 @@ import type { WebhookDeliveryJobData } from './types.js';
 export const WEBHOOK_DELIVERY_QUEUE_NAME = 'webhook-delivery-queue';
 export const WEBHOOK_DEAD_LETTER_QUEUE_NAME = 'webhook-dead-letter-queue';
 
+/**
+ * Jittered exponential backoff (Issue #1129): BullMQ's built-in exponential
+ * strategy with a 50% jitter factor so concurrent retries to the same
+ * destination don't stampede in lockstep. `delay` is the base delay in ms.
+ */
+export const WEBHOOK_BACKOFF_DELAY_MS = Number(process.env.WEBHOOK_BACKOFF_DELAY_MS || '1000');
+
+export const WEBHOOK_MAX_ATTEMPTS = Number(process.env.WEBHOOK_MAX_ATTEMPTS || '5');
+
+export const WEBHOOK_BACKOFF_JITTER = 0.5;
+
 const createDefaultJobOptions = () => ({
-  attempts: Number(process.env.WEBHOOK_MAX_ATTEMPTS || '5'),
+  attempts: WEBHOOK_MAX_ATTEMPTS,
   backoff: {
     type: 'exponential' as const,
-    delay: Number(process.env.WEBHOOK_BACKOFF_DELAY_MS || '1000'),
+    delay: WEBHOOK_BACKOFF_DELAY_MS,
+    jitter: WEBHOOK_BACKOFF_JITTER,
   },
   removeOnComplete: {
     age: 24 * 60 * 60,