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/models/.pi-map.index.md

@@ -0,0 +1,22 @@
+# backend/src/media_library_viewer_api/models (index)
+dir: backend/src/media_library_viewer_api/models
+
+## role
+Defines Pydantic data models for request/response validation across backup management, service registry, and dashboard widget APIs.
+## parent
+index: backend/src/media_library_viewer_api/.pi-map.index.md
+map: backend/src/media_library_viewer_api/.pi-map.md
+## children
+-
+## files
+- backups.py
+- services.py
+- widgets.py
+## links
+index: backend/src/media_library_viewer_api/models/.pi-map.index.md
+map: backend/src/media_library_viewer_api/models/.pi-map.md
+## workflows
+- change models behavior
+  read: backups.py, services.py, widgets.py
+## dirty
+-

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

@@ -0,0 +1,29 @@
+# backend/src/media_library_viewer_api/models
+dir: backend/src/media_library_viewer_api/models
+
+index: backend/src/media_library_viewer_api/models/.pi-map.index.md
+
+## role
+Defines Pydantic data models for request/response validation across backup management, service registry, and dashboard widget APIs.
+## files
+- backups.py | Defines Pydantic data models for backup system API requests and responses including reports, jobs, runs, alerts, and dashboard summaries. | exp: class:BackupReportRequest, class:BackupJobResponse, class:BackupRunResponse, class:BackupAlertResponse, class:BackupDashboardSummary | dep: datetime, typing, pydantic
+- services.py | Defines Pydantic models for a service registry API, including validation to prevent credential keys in non-secret configuration. | exp: class:ServiceInstanceInput, class:ServiceInstance, class:SecretFieldInfo, class:WidgetKindInfo, class:ServiceTypeInfo, func:_validate_config_keys(config: dict[str, Any]) → dict[str, Any], call:isinstance, call:value.items, call:key.lower, call:_check, raise:ValueError, func:_check(value: Any) → None, call:isinstance, call:value.items, call:key.lower, call:_check, raise:ValueError | dep: typing, pydantic
+- widgets.py | Defines Pydantic models for a dashboard widget system with validation to prevent secrets/credentials in widget configuration. | exp: class:_WidgetInstanceBase, class:WidgetInstanceInput, class:WidgetInstance, class:BuiltinWidgetKindInfo, class:WidgetDataResponse, func:_looks_secret(value: Any) → bool, call:isinstance, call:value.strip, call:value.lower, call:value.startswith, call:len, call:lowered.isalnum, func:_validate_config_keys(config: dict[str, Any]) → dict[str, Any], call:config.items, call:key.lower, call:_looks_secret, call:isinstance, call:_validate_config_keys, raise:ValueError | dep: typing, pydantic
+## arch
+Pydantic-based model layer implementing data validation, serialization contracts, and custom validators enforcing security constraints (e.g., blocking credentials in non-secret configs).
+## tags
+widget, backup, instance, response, info, call:isinstance, call:, service
+## symbols
+- BackupReportRequest
+- BackupJobResponse
+- BackupRunResponse
+- BackupAlertResponse
+- BackupDashboardSummary
+- ServiceInstanceInput
+- ServiceInstance
+- SecretFieldInfo
+## workflows
+- change models behavior
+  read: backups.py, services.py, widgets.py
+## dirty
+-