docs(admin): settings page — every knob, what it does, and its range

The ninth page and the core of the owner's admin-docs brief: all 29
registry settings plus Base URL, documented setting-by-setting to the
admin tier — key, type, default, valid range, live-vs-restart, and
prose for what it does and what it means when that's confusing
(extractor-version as the gradual backfill lever vs. force rescan;
password rules apply at next set, never retroactively; tombstone TTL
as offline-device protection; rate limits as throttling behavior;
worker pool sizing honesty for small servers). Grouped exactly like
the page renders them, with the three save paths explained up front.
Ranges/defaults/flags from SettingDefaults; exclusion of the three
dedicated-UI keys from the tunables verified in the page handler.
Backfills the Settings links into dashboard.md and users.md now that
the target exists. Glossary gains Save Settings, Save Scan Settings,
both Tunable bucket headings, and Default Timezone.

docs/admin/dashboard.md

@@ -29,11 +29,11 @@ user can see.
 
 The **File Watcher** card shows how many libraries are currently under
 live watch for file changes. The watcher exists when **Auto-Scan** is
-enabled on the Settings page (Settings → Scanning); the number updates on
-page load.
+enabled on the Settings page; the number updates on page load.
 
 - Watching **0** libraries usually means auto-scan is off — turn it on in
-  Settings → Scanning, and the watcher arms on the next server start.
+  [Settings](settings.md) → Scanning, and the watcher arms on the next
+  server start.
 - Watched libraries get shallow scans as files change, plus a periodic
   poll on the scan interval — see [Scanning](scanning.md) for exactly what
   those do.