web.poll_feed_task: use URL as id since some feeds use non-URL (eg tag URI) ids

example: https://www.producthunt.com/feed, Atom. excerpt:

```xml
<?xml version="1.0" encoding="UTF-8"?>
<feed xml:lang="en-US" xmlns="http://www.w3.org/2005/Atom">
  <id>tag:www.producthunt.com,2005:/feed</id>
  <link rel="alternate" type="text/html" href="https://www.producthunt.com"/>
  <link rel="self" type="application/atom+xml" href="https://www.producthunt.com/feed"/>
  <title>Product Hunt — The best new products, every day</title>
  <updated>2024-01-10T02:57:06-08:00</updated>
  <entry>
    <id>tag:www.producthunt.com,2005:Post/432225</id>
    <published>2023-12-31T18:15:55-08:00</published>
    <updated>2024-01-10T12:54:42-08:00</updated>
    <link rel="alternate" type="text/html" href="https://www.producthunt.com/posts/seemless"/>
    <title>Seemless</title>
    ...
```

fixes https://console.cloud.google.com/errors/detail/CKLuk-v4x8X0NQ;time=P30D?project=bridgy-federated

tests/test_web.py

@@ -1779,8 +1779,8 @@ class WebTest(TestCase):
         feed = """\
 <?xml version="1.0" encoding="UTF-8"?>
 <entry xmlns="http://www.w3.org/2005/Atom">
-<uri>https://user.com/post</uri>
-<content>I hereby ☕ post</content>
+  <link rel="alternate" type="text/html" href="https://user.com/post" />
+  <content>I hereby ☕ post</content>
 </entry>
 """
         mock_get.return_value = requests_response(
@@ -1930,6 +1930,58 @@ class WebTest(TestCase):
         ))
         assert Object.get_by_id('https://user.com/post')
 
+    @patch('oauth_dropins.webutil.appengine_config.tasks_client.create_task')
+    def test_poll_feed_use_url_as_id(self, mock_create_task, mock_get, _):
+        common.RUN_TASKS_INLINE = False
+        self.user.obj.mf2 = ACTOR_MF2_REL_FEED_URL
+        self.user.obj.put()
+
+        feed = """\
+<?xml version="1.0" encoding="UTF-8"?>
+<entry xmlns="http://www.w3.org/2005/Atom">
+  <id>tag:user.com,2999:abc</id>
+  <link rel="alternate" type="text/html" href="https://user.com/post" />
+  <content>I hereby ☕ post</content>
+</entry>
+"""
+        mock_get.return_value = requests_response(
+            feed, headers={'Content-Type': atom.CONTENT_TYPE})
+
+        got = self.post('/queue/poll-feed', data={'domain': 'user.com'})
+        self.assertEqual(200, got.status_code)
+        self.assertEqual(NOW, self.user.key.get().last_polled_feed)
+
+        mock_get.assert_has_calls((
+            self.req('https://foo/feed'),
+        ))
+        obj = self.assert_object('https://user.com/post',
+                                 users=[self.user.key],
+                                 source_protocol='web',
+                                 status='new',
+                                 atom=feed,
+                                 our_as1={
+                                     'objectType': 'activity',
+                                     'verb': 'post',
+                                     'id': 'https://user.com/post',
+                                     'url': 'https://user.com/post',
+                                     'actor': {'id': 'https://user.com/'},
+                                     'object':{
+                                         'objectType': 'note',
+                                         'id': 'https://user.com/post',
+                                         'url': 'https://user.com/post',
+                                         'author': {'id': 'https://user.com/'},
+                                         'content': 'I hereby ☕ post',
+                                     },
+                                     'feed_index': 0,
+                                 },
+                                 type='post',
+                                 object_ids=['https://user.com/post'],
+                                 labels=['user', 'activity'],
+                                 )
+        self.assert_task(mock_create_task, 'receive', '/queue/receive',
+                         obj=obj.key.urlsafe(),
+                         authed_as='user.com')
+
     def test_poll_feed_fails(self, mock_get, _):
         common.RUN_TASKS_INLINE = False
         self.user.obj.mf2 = ACTOR_MF2_REL_FEED_URL

web.py

@@ -668,9 +668,13 @@ def poll_feed_task():
     for i, activity in enumerate(activities):
         # default actor and author to user
         activity.setdefault('actor', {}).setdefault('id', user.profile_id())
-        activity.setdefault('object', {})\
-                .setdefault('author', {})\
-                .setdefault('id', user.profile_id())
+        obj = activity.setdefault('object', {})
+        obj.setdefault('author', {}).setdefault('id', user.profile_id())
+
+        # use URL as id since some feeds use non-URL (eg tag URI) ids
+        for elem in obj, activity:
+            if url := elem.get('url'):
+                elem['id'] = elem['url']
 
         logger.info(f'Converted to AS1: {json_dumps(activity, indent=2)}')