The exact-equality assert on the line above (requested_url == WEBHOOK_BASE)
already implies the URL has no trailing slash, so the endswith check adds
nothing.
_join_integration_url built urljoin(base + '/', '') for a bare '/'
path — the minimum execute_api_call accepts — so every request against
a POST-to-base integration went to base_url + '/'. Discord webhook
URLs 404 ('Unknown Webhook') on the trailing-slash variant, which made
the integration look broken even though the stored base URL was
correct.
Resolve a bare '/' (or empty) path to the base URL itself and keep all
other paths joining exactly as before, including deliberate trailing
slashes inside non-empty paths (linkding /api/tags/, Home Assistant
/api/). The reminder webhook sender and the discord_webhook
connectivity test already posted to the bare base URL; execute_api_call
was the remaining path that re-added the slash.
Fixes#5138
Harshit Ojha