fix: split HTTP connect/read timeouts (Jellyfin build + qBit stats)

Every HTTP client passed an integer timeout to requests, applying the same
value to BOTH connect and read phases. A slow Jellyfin /Items page or qBit
/sync/maindata blew through the 10s read budget → ReadTimeoutError. Split
into a (connect=5s, read=60s default) tuple via shared http_timeout() helper.
The media index build worker uses a 180s read floor. Existing services with
low timeout_seconds benefit from bumping to 60+.

backend/src/media_library_viewer_api/integrations/alertmanager.py

@@ -25,7 +25,7 @@ class AlertmanagerConfig(ServiceConfigBase):
     """Non-secret Alertmanager connection config."""
 
     base_url: ServiceBaseUrl
-    timeout_seconds: int = 5
+    timeout_seconds: int = 15
 
 
 class AlertmanagerAlertsWidgetConfig(WidgetConfigBase):
@@ -83,7 +83,7 @@ def test_connection(
     """GET /api/v2/status with optional bearer auth."""
     try:
         base_url = str(config.get("base_url") or "").rstrip("/")
-        timeout = int(config.get("timeout_seconds") or 5)
+        timeout = int(config.get("timeout_seconds") or 15)
         headers: dict[str, str] = {}
         api_key = str(secrets.get("api_key") or "")
         if api_key: