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/developer/ui-controls.md

@@ -308,6 +308,26 @@ label: Edit User Profile
 where: Admin → Users → a user card → Edit
 does: The admin edit modal — profile fields, role, and Change Password without requiring the user's current password.
 
+label: Save Settings
+where: Admin → Settings → top card
+does: Commits the Base URL and Default Timezone together.
+
+label: Save Scan Settings
+where: Admin → Settings → Scanning card
+does: Commits Auto-Scan and the Scan Interval.
+
+label: Tunable Settings — Live
+where: Admin → Settings → section heading
+does: Settings that apply immediately on save, grouped by area.
+
+label: Tunable Settings — Restart Required
+where: Admin → Settings → section heading
+does: Settings whose saves take effect only after the server restarts; the bucket is marked in amber.
+
+label: Default Timezone
+where: Admin → Settings → System Defaults card
+does: The timezone used for any user who hasn't chosen their own; governs how dates and times render for them.
+
 label: Resume rewind
 where: audio player → player settings; profile settings
 does: Rewinds playback a configured few seconds when you resume.