fix(integrations): don't append a trailing slash when api_call path is '/'

_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

src/integrations.py

@@ -216,7 +216,14 @@ def _normalize_integration_base_url(base_url: Any) -> str:
 
 
 def _join_integration_url(base_url: str, path: str) -> str:
-    return urljoin(base_url.rstrip("/") + "/", path.lstrip("/"))
+    base = base_url.rstrip("/")
+    rel = path.lstrip("/")
+    if not rel:
+        # A bare "/" must resolve to the base URL itself, not base + "/".
+        # POST-to-base integrations (e.g. Discord webhooks) 404 on the
+        # trailing-slash variant of their URL.
+        return base
+    return urljoin(base + "/", rel)
 
 
 def load_integrations() -> List[Dict[str, Any]]: