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/base.py

@@ -16,9 +16,37 @@ map. There is no runtime plugin loading.
 from __future__ import annotations
 
 from dataclasses import dataclass, field
-from typing import Any
+from typing import Annotated, Any
 
-from pydantic import BaseModel
+from pydantic import BaseModel, BeforeValidator, Field
+
+
+def _validate_service_base_url(value: Any) -> str:
+    """Require an absolute http(s) URL for service ``base_url`` fields.
+
+    Relative hosts (e.g. ``grafana.example.com``) break downstream HTTP clients
+    because ``requests`` treats them as relative paths, so we fail fast with a
+    clear error instead of letting the call silently malfunction.
+    """
+    if not isinstance(value, str):
+        raise ValueError("base_url must be a string starting with http:// or https://")
+    text = value.strip()
+    if not text:
+        raise ValueError("base_url must not be empty")
+    lowered = text.lower()
+    if not (lowered.startswith("http://") or lowered.startswith("https://")):
+        raise ValueError("base_url must start with http:// or https:// (include the schema)")
+    return text
+
+
+#: Shared annotated type for service ``base_url`` fields. applying the validator
+#: uniformly across every integration so missing schemas are rejected at the
+#: config boundary with a helpful message.
+ServiceBaseUrl = Annotated[
+    str,
+    Field(description="Absolute URL including the http:// or https:// schema."),
+    BeforeValidator(_validate_service_base_url),
+]
 
 
 class ServiceConfigBase(BaseModel):
@@ -26,6 +54,9 @@ class ServiceConfigBase(BaseModel):
 
     Subclass this in each integration module and declare the connection fields.
     The JSON schema is derived via ``model_json_schema()`` and exposed to the UI.
+
+    Connection URLs should use the :data:`ServiceBaseUrl` type so the
+    ``http(s)://`` schema is enforced consistently across integrations.
     """