feat(services): backend service registry foundation (encryption, definitions, CRUD)

PR 1 of 4 for the runtime service registry change.

- Add Fernet encryption helper (services/secrets.py) with a required
  MANAGE_ENCRYPTION_KEY; validate it on startup.
- Add closed integrations/ registry with Pydantic config + widget-config
  definitions for grafana, prometheus, jellyfin, nextcloud, and ssh_tasks.
- Add services + service_task_runs tables and SettingsStore CRUD with
  cascade-delete (defensive until widgets carry service_id).
- Add /api/services/types and /api/services/instances CRUD (encrypted secrets,
  secrets_set flags only; never plaintext).
- Declare cryptography as a direct dependency.
- Require MANAGE_ENCRYPTION_KEY in compose + .env.example + README.
- Add 25 backend tests (registry, encryption, CRUD, cascade, task-run history).

Verification: ruff clean; pytest 225 passed; frontend lint/build green.

backend/src/media_library_viewer_api/integrations/prometheus.py

@@ -0,0 +1,45 @@
+"""Prometheus service definition."""
+
+from __future__ import annotations
+
+from media_library_viewer_api.integrations.base import (
+    SecretField,
+    ServiceConfigBase,
+    ServiceDefinition,
+    WidgetConfigBase,
+    widget_kind,
+)
+
+
+class PrometheusConfig(ServiceConfigBase):
+    """Non-secret Prometheus connection config."""
+
+    base_url: str
+    timeout_seconds: int = 10
+
+
+class PrometheusMetricWidgetConfig(WidgetConfigBase):
+    """A PromQL instant query rendered as a metric."""
+
+    promql: str
+
+
+DEFINITION = ServiceDefinition(
+    service_type="prometheus",
+    name="Prometheus",
+    description="Metrics storage and PromQL queries.",
+    config_model=PrometheusConfig,
+    secret_fields=[
+        SecretField(key="api_key", label="API key", helper="Optional bearer token"),
+    ],
+    widget_kinds=[
+        widget_kind(
+            kind="metric",
+            name="Metric",
+            description="Instant query result rendered as a metric.",
+            model_cls=PrometheusMetricWidgetConfig,
+            default_config={"promql": ""},
+            refresh_interval_ms=30_000,
+        ),
+    ],
+)