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.

shopdb/plugins/base.py

@@ -104,6 +104,28 @@ class BasePlugin(ABC):
         """
         return []
 
+    def get_settings_defaults(self) -> List[Dict]:
+        """Declare the Setting rows this plugin owns.
+
+        The framework seeds these at install, at enable, and on every
+        `flask plugin upgrade-all`, so a setting added in a later plugin
+        version reaches a site that installed an earlier one. Declaring a key
+        here is also what tells the settings API which category and type the
+        key belongs to, so a save never has to invent one.
+
+        Each entry is a dict:
+          key         - the Setting key
+          value       - default value (string form)
+          valuetype   - 'string' | 'boolean' | 'integer' | 'json'
+          category    - grouping the plugin's settings page filters on
+          description - what the setting does
+          public      - True if an UNAUTHENTICATED caller may read it (kiosk
+                        and print pages render before login); default False.
+                        Never mark a credential or an integration URL public.
+        Return [] (default) if the plugin owns no settings.
+        """
+        return []
+
     def get_setting(self, key: str, default=None):
         """Read a plugin-scoped setting from the core Setting store.