chore: archive observability-service-registry, track pi-map artifacts

- Archive the completed observability-service-registry SDD change into
  openspec/changes/archive/ (delivered across 5 slices; only
  jellyfin-service-registry remains active).
- Stop ignoring .pi-map.md / .pi-map.index.md so the navigation maps are
  versioned alongside the code, and add the regenerated map pairs repo-wide.

backend/src/media_library_viewer_api/widgets/.pi-map.index.md

@@ -0,0 +1,22 @@
+# backend/src/media_library_viewer_api/widgets (index)
+dir: backend/src/media_library_viewer_api/widgets
+
+## role
+Provides widget definitions, schemas, and data source adapters for rendering configurable dashboard widgets from both built-in and external service data.
+## parent
+index: backend/src/media_library_viewer_api/.pi-map.index.md
+map: backend/src/media_library_viewer_api/.pi-map.md
+## children
+-
+## files
+- __init__.py
+- builtin.py
+- sources.py
+## links
+index: backend/src/media_library_viewer_api/widgets/.pi-map.index.md
+map: backend/src/media_library_viewer_api/widgets/.pi-map.md
+## workflows
+- change widgets behavior
+  read: __init__.py, builtin.py, sources.py
+## dirty
+-

backend/src/media_library_viewer_api/widgets/.pi-map.md

@@ -0,0 +1,29 @@
+# backend/src/media_library_viewer_api/widgets
+dir: backend/src/media_library_viewer_api/widgets
+
+index: backend/src/media_library_viewer_api/widgets/.pi-map.index.md
+
+## role
+Provides widget definitions, schemas, and data source adapters for rendering configurable dashboard widgets from both built-in and external service data.
+## files
+- __init__.py | Marks the directory as a Python package for the widget subsystem.
+- builtin.py | Defines built-in widget kinds that don't require external services, providing their configurations, schemas, and validation. | exp: class:StaticConfig, func:get_builtin_widget_kind(kind: str) → WidgetKind | None, call:BUILTIN_WIDGET_KINDS.get, func:is_builtin_kind(kind: str) → bool, func:builtin_widget_kind_models() → dict[str, type], call:Field, func:validate_builtin_config(kind: str, config: dict[str, Any]) → dict[str, Any], call:builtin_widget_kind_models, call:models.get, call:dict, call:model_cls.model_validate(config or {}).model_dump | dep: typing, media_library_viewer_api.integrations.base, pydantic
+- sources.py | Defines widget source adapters that fetch and transform data from various external services (Grafana, Prometheus, Jellyfin, etc.) into dashboard widget payloads. | exp: class:ServiceRecord, class:WidgetSource, method:fetch(self, service: ServiceRecord | None, widget_kind: str, config: dict[str, Any]) → dict[str, Any], class:BackupsWidgetSource, method:fetch(self, service: ServiceRecord | None, widget_kind: str, config: dict[str, Any]) → dict[str, Any], call:get_settings_store, call:build_backup_dashboard_summary, call:summary.model_dump, call:logger.exception, class:StaticWidgetSource, method:fetch(self, service: ServiceRecord | None, widget_kind: str, config: dict[str, Any]) → dict[str, Any], call:config.get, class:GrafanaWidgetSource, method:fetch(self, service: ServiceRecord | None, widget_kind: str, config: dict[str, Any]) → dict[str, Any], call:str(service.config.get("base_url") or "").rstrip, call:service.config.get, call:config.get, call:logger.exception, class:PrometheusWidgetSource, method:fetch(self, service: ServiceRecord | None, widget_kind: str, config: dict[str, Any]) → dict[str, Any], call:str(service.config.get("base_url") or "").rstrip, call:service.config.get, call:int, call:config.get, call:asyncio.wait_for, call:asyncio.to_thread, call:response.raise_for_status, call:response.json, call:payload.get, call:logger.exception, class:AlertmanagerWidgetSource, method:fetch(self, service: ServiceRecord | None, widget_kind: str, config: dict[str, Any]) → dict[str, Any], call:str(service.config.get("base_url") or "").rstrip, call:service.config.get, call:int, call:config.get, call:service.secrets.get, call:asyncio.wait_for, call:asyncio.to_thread, call:response.raise_for_status, call:response.json, call:payload.get, call:isinstance, call:summarize_alerts, call:logger.exception, class:JellyfinWidgetSource, method:fetch(self, service: ServiceRecord | None, widget_kind: str, config: dict[str, Any]) → dict[str, Any], call:str, call:service.config.get, call:service.secrets.get, call:int, call:asyncio.wait_for, call:asyncio.to_thread, call:_map_sessions_to_activity_rows, call:logger.exception, class:SshTaskWidgetSource, method:fetch(self, service: ServiceRecord | None, widget_kind: str, config: dict[str, Any]) → dict[str, Any], call:get_settings_store, call:config.get, call:store.get_task, call:task.get, call:int, call:service.config.get, call:asyncio.wait_for, call:asyncio.to_thread, call:_record_timeout, call:logger.exception, func:build_service_record(store: SettingsStore, service_row: dict[str, Any]) → ServiceRecord, call:ServiceRecord, call:service_row.get, call:decrypt_secrets, call:bool, func:_record_timeout(service: ServiceRecord | None, config: dict[str, Any], timeout: int) → None, call:get_settings_store, call:store.record_service_task_run, call:str, call:config.get, call:logger.exception, func:get_service_adapter(service_type: str) → WidgetSource | None, call:SERVICE_ADAPTERS.get, func:get_builtin_adapter(kind: str) → WidgetSource | None, call:BUILTIN_ADAPTERS.get | dep: asyncio, logging, dataclasses, typing, requests, media_library_viewer_api.clients.jellyfin, media_library_viewer_api.domain.dashboard, media_library_viewer_api.integrations.alertmanager, media_library_viewer_api.services.settings_store, media_library_viewer_api.services.task_runner, media_library_viewer_api.services.secrets
+## arch
+Adapter pattern with modular source integrations that normalize heterogeneous external API responses into a unified widget payload format, complemented by schema-validated built-in widget configurations.
+## tags
+widget, source, fetch, call:logger.exception, builtin, call:config.get, record, call:str
+## symbols
+- StaticConfig
+- ServiceRecord
+- WidgetSource
+- BackupsWidgetSource
+- StaticWidgetSource
+- GrafanaWidgetSource
+- PrometheusWidgetSource
+- AlertmanagerWidgetSource
+## workflows
+- change widgets behavior
+  read: __init__.py, builtin.py, sources.py
+## dirty
+-