Show the kiosk label prefix, and let a plugin declare the settings it owns

Three defects, all found on printedparts_label_prefix, all one root cause:
nothing in the framework knew that setting existed.

The parts kiosk runs logged out. An unauthenticated read of a setting is
limited to an allowlist, the key was not on it, so the kiosk got a 404 and
fell back to no prefix. An admin previewing the same page while logged in saw
the prefix, which is why it looked like it worked.

The same setting also looked like it would not save. The row did not exist on
a site that installed the plugin before the setting was added, so the first
save created it - under the placeholder category the settings API uses for
keys it does not recognise, where the plugin's settings page, which lists by
category, could no longer see it. The value was in the database the whole
time.

And the row was missing in the first place because seeding ran from
on_install / on_enable, which fire only on a state transition. Neither runs
again on an upgrade, so a setting added in a later plugin version never
reached a site that installed an earlier one. The comment claiming enable ran
every upgrade cycle was simply wrong.

A plugin now declares the settings it owns in get_settings_defaults(): key,
default, type, category, description, and whether a logged-out page may read
it. The framework seeds declared keys at install, at enable, and on every
flask plugin upgrade-all; files a first-time write under the declared
category; re-homes any row left in the placeholder category, value untouched;
and answers an anonymous read for keys marked public. Core carries no list of
any plugin's keys.

Contract 0.16.0 (additive optional hook). printedparts and printers move to
the hook and floor their core_version at 0.16.0. The dev database had two rows
in the misfiled state (printedparts_alert_email, employee_db_host); the first
repairs itself on the next upgrade pass.

CHANGELOG.md

@@ -10,6 +10,35 @@ ADR-007 and ADR-002.
 
 ## [Unreleased]
 
+### Fixed
+
+- The 3D parts kiosk label prefix never appeared on the kiosk. The kiosk runs
+  logged out, and an unauthenticated read of a setting is limited to an
+  allowlist the key was not on, so the kiosk got a 404 and fell back to no
+  prefix. An admin previewing the same page while logged in saw it, which is
+  why it looked like it worked. A plugin now declares which of its settings a
+  logged-out page may read.
+- The same setting also looked like it would not save. The row did not exist on
+  a site that installed the plugin before the setting was added, so the first
+  save created it - under a placeholder category, where the settings page, which
+  lists by category, could no longer see it. The value was in the database the
+  whole time. A first-time save now files the key under the category its owner
+  declares. Any row already misfiled is repaired in place on upgrade, value
+  untouched. The 3D parts alert email and the employee directory host were in
+  that state.
+- Settings added in a later plugin version never reached a site that installed
+  an earlier one. The seeding ran from `on_install` / `on_enable`, which fire
+  only on a state transition, so neither ran again on an upgrade - and the
+  comment claiming enable ran every upgrade cycle was wrong.
+  `flask plugin upgrade-all` now seeds missing declared settings, without
+  touching values a site has configured.
+
+### Added
+
+- Plugin contract 0.16.0: `get_settings_defaults()` lets a plugin declare the
+  settings it owns (key, default, type, category, description, and whether an
+  unauthenticated caller may read it). See `docs/PLUGIN-HOOKS.md`.
+
 ## [0.8.1] - 2026-08-05
 
 Everything here shipped after v0.8.0 was tagged the same morning, driven by two