fix(webhooks): write delivery timestamps as ISO strings

Applies the repo's documented Jest+SQLite guidance (CLAUDE.md) to the webhook
delivery path, which was the last one still passing raw Date objects into
knex writes. Under jest those store as the literal string "[object Object]",
so next_retry_at came back NaN and the retry/backoff test could not assert on
it. Production (PG, and SQLite outside jest) was unaffected.

Convert the timestamp writes -- and the `next_retry_at <=` due comparison,
which has to stay type-consistent with them -- to .toISOString(), matching
the existing precedent in downloadJobService.js.

Refs testplan REPORT.md #22 (Part 1.2.01).

backend/__tests__/integration/webhookDelivery.test.js

@@ -146,8 +146,8 @@ describe('webhook delivery worker (#327)', () => {
         payload: JSON.stringify({ id: 'd1', type: 'event.published', data: {} }),
         attempt_count: 4,
         status: 'pending',
-        next_retry_at: new Date(),
-        created_at: new Date(),
+        next_retry_at: new Date().toISOString(),
+        created_at: new Date().toISOString(),
       });
       await __test.tick();
 
@@ -190,8 +190,8 @@ describe('webhook delivery worker (#327)', () => {
         payload: JSON.stringify({ id: 'd1', type: 'event.published', data: {} }),
         attempt_count: 0,
         status: 'pending',
-        next_retry_at: new Date(),
-        created_at: new Date(),
+        next_retry_at: new Date().toISOString(),
+        created_at: new Date().toISOString(),
       });
       await __test.tick();
 
@@ -214,8 +214,8 @@ describe('webhook delivery worker (#327)', () => {
         payload: JSON.stringify({ id: 'd1', type: 'event.published', data: {} }),
         attempt_count: 0,
         status: 'pending',
-        next_retry_at: new Date(),
-        created_at: new Date(),
+        next_retry_at: new Date().toISOString(),
+        created_at: new Date().toISOString(),
       });
       await __test.tick();