fix: let the download reach the PO token provider (closes #1064)

The image ships yt-dlp's bgutil PO token provider and starts it on
loopback, where the plugin dials it at http://127.0.0.1:4416. Since
482381d scoped the connect-time allowance to the configured proxy, the
download subprocess could no longer resolve it:

    Refusing to connect to non-global address for host '127.0.0.1'

which surfaces as the plugin's "Error reaching GET .../ping". Metadata
extraction runs in the main process and installs no guard, so titles kept
resolving while the download itself ran without a token — and YouTube
increasingly answers those with 403.

The allowance already had the right shape for this; it was just named for
its only user. Endpoints the operator or the image configured are now
allowed as a class: install_socket_guard takes service_urls alongside
proxy_urls, and ytdl derives them from the bundled default plus any
base_url set through the youtubepot-bgutilhttp (or the deprecated youtube
getpot_bgutil_baseurl) extractor argument. The bundled server runs either
way, so it stays allowed when a base URL is configured.

Matching stays exact host:port on the configured string, so nothing else
on loopback opens up: a hostile media URL naming the endpoint reaches a
token server with two endpoints and nothing worth reading.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/tests/test_ytdl_utils.py

@@ -77,6 +77,7 @@ from ytdl import (
     MusicMetadataPreProcessor,
     _compact_persisted_entry,
     _convert_srt_to_txt_file,
+    _pot_provider_urls,
     _AlbumArtistPostProcessor,
     _resolve_outtmpl_fields,
     _sanitize_entry_for_pickle,
@@ -1071,5 +1072,44 @@ class ShortTitleForFailedUrlTests(unittest.TestCase):
         self.assertEqual(_short_title_for_failed_url(malformed), malformed)
 
 
+class PotProviderUrlsTests(unittest.TestCase):
+    """#1064: the connect-time guard must let the download reach the PO token
+    provider, so it has to know every endpoint yt-dlp might dial for one."""
+
+    def test_bundled_provider_by_default(self):
+        self.assertEqual(_pot_provider_urls({}), ("http://127.0.0.1:4416",))
+
+    def test_configured_base_url_is_added(self):
+        urls = _pot_provider_urls({
+            "extractor_args": {"youtubepot-bgutilhttp": {"base_url": ["http://pot:4416"]}},
+        })
+        # The bundled server runs regardless, so both stay reachable.
+        self.assertEqual(urls, ("http://127.0.0.1:4416", "http://pot:4416"))
+
+    def test_deprecated_base_url_arg_is_honoured(self):
+        urls = _pot_provider_urls({
+            "extractor_args": {"youtube": {"getpot_bgutil_baseurl": ["http://pot:4416"]}},
+        })
+        self.assertEqual(urls, ("http://127.0.0.1:4416", "http://pot:4416"))
+
+    def test_unrelated_extractor_args_are_ignored(self):
+        urls = _pot_provider_urls({
+            "extractor_args": {"youtube": {"player_client": ["web"]}},
+        })
+        self.assertEqual(urls, ("http://127.0.0.1:4416",))
+
+    def test_malformed_extractor_args_do_not_raise(self):
+        # YTDL_OPTIONS is operator-supplied JSON and reaches here unvalidated.
+        for opts in (
+            {"extractor_args": None},
+            {"extractor_args": "youtube:player_client=web"},
+            {"extractor_args": {"youtubepot-bgutilhttp": "http://pot:4416"}},
+            {"extractor_args": {"youtubepot-bgutilhttp": {"base_url": []}}},
+        ):
+            with self.subTest(opts=opts):
+                self.assertEqual(_pot_provider_urls(opts), ("http://127.0.0.1:4416",))
+
+
 if __name__ == "__main__":
+
     unittest.main()