Enforce http(s) schema on service base_url fields

Add a shared ServiceBaseUrl type (BeforeValidator + Field description) in
integrations/base.py and apply it to base_url across all six service configs
(grafana, prometheus, alertmanager, jellyfin, jellyseerr, nextcloud). Missing
http:// or https:// schema now fails fast with a clear 422 instead of breaking
HTTP clients silently. Tests cover reject/accept cases; REQUIREMENTS updated.

backend/src/media_library_viewer_api/integrations/alertmanager.py

@@ -6,6 +6,7 @@ from typing import Any
 
 from media_library_viewer_api.integrations.base import (
     SecretField,
+    ServiceBaseUrl,
     ServiceConfigBase,
     ServiceDefinition,
     WidgetConfigBase,
@@ -16,7 +17,7 @@ from media_library_viewer_api.integrations.base import (
 class AlertmanagerConfig(ServiceConfigBase):
     """Non-secret Alertmanager connection config."""
 
-    base_url: str
+    base_url: ServiceBaseUrl
     timeout_seconds: int = 5