feat(prometheus): the Prometheus dashboard widget, target + template (#5691)

#5691 "Generic Prometheus dashboard widget: pick a target and a template",
step 3 of milestone 508 "Prometheus widget templates".

- /plugins/prometheus/widget draws one target with one template in widget
  shape: the verdict (led by "isn't answering" when the target is down),
  then each block with problems first, figures beside a share bar or the
  last 24h of bars, and the healthy rest folding to "+N more healthy" past
  six. A missing target or template, or one that no longer checks, says so.
- library.py: bundled templates from plugins/prometheus/widgets/*.yaml,
  the picker's target and template lists, and the default title.
- widgets/fabled_archives.yaml: the first bundled template, against the
  series Archives reports (refined in #5693).
- Dashboard: a widget setting may take `options_from` (read when the picker
  opens) with an `empty` sentence; `default_title`; a Details link may name
  settings ("/plugins/prometheus/targets/{target}"). Prometheus is grouped
  with the integrations.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/prometheus/widgets/fabled_archives.yaml

@@ -0,0 +1,55 @@
+# Fabled Archives, from its /metrics on the LAN-only backup listener (:8081).
+# Written against what Archives reports; see docs/plugins/prometheus.md.
+name: Fabled Archives
+description: Backup targets, replication jobs, storage, integrity and the version running.
+version: 1
+expects: fabledarchives_
+blocks:
+  - states:
+      title: Backup targets
+      metric: fabledarchives_backup_target_state
+      name: target
+      healthy: [fresh]
+      quiet: [disabled, waiting]
+      warning: [late]
+      columns:
+        - {label: Last backup, metric: fabledarchives_backup_target_last_backup_timestamp_seconds}
+        - {label: Stored, metric: fabledarchives_backup_target_stored_bytes}
+  - states:
+      title: Replication
+      metric: fabledarchives_replication_job_state
+      name: job
+      healthy: [ok, running]
+      warning: [late, warning, never]
+      columns:
+        - {label: Next run, metric: fabledarchives_replication_job_next_run_timestamp_seconds}
+  - values:
+      title: Storage and integrity
+      items:
+        - label: Disk free
+          metric: fabledarchives_data_disk_free_bytes
+          of: fabledarchives_data_disk_size_bytes
+          warn_below: 10%
+          crit_below: 5%
+          sparkline: true
+        - label: Files checked
+          metric: fabledarchives_files_checked
+          of: fabledarchives_files
+          unit: count
+        - label: Malware held
+          metric: fabledarchives_malware_held_files
+          unit: count
+          warn_above: 0
+        - label: Notification channels failing
+          metric: fabledarchives_notification_channels_failing
+          unit: count
+          warn_above: 0
+  - table:
+      title: Stored per person
+      metric: fabledarchives_person_stored_bytes
+      rows: person
+      columns: kind
+  - info:
+      title: Running
+      metric: fabledarchives_build_info
+      labels: [version, channel, commit]