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/users.md

@@ -16,7 +16,7 @@ flow.
 **+ Create User** opens the creation form (it stays hidden until clicked,
 so the page doesn't shout account creation at you — and so admins can
 still create accounts even when public self-registration is switched off
-on the Settings page).
+in [Settings](settings.md) → Public Registration).
 
 Fields: **Username** (3–50 characters), **Email**, **Password** plus a
 confirm, optional **First name** / **Last name**, and a **Role** of User