Anyone not signed in holds the guest role

Whether somebody with no account may see something was being answered one
feature at a time: a securesend_allow_guests setting, an enable flag per kiosk,
and very nearly a third for guides - each with its own settings page, its own
vocabulary and its own hand-written branch inside a route. Every one of them
answered a permissions question, and this app already administers permissions
per role.

So a caller with no token is not nobody: they hold the seeded `guest` role.
require_permission consults it when there is no identity, which is the entire
mechanism. The role is seeded holding NOTHING, so an upgrade opens no door; a
site opens one by ticking a box in the same grid it uses for every other role,
and closes it the same way. Revocation is read on every request rather than
cached, because taking guest access away is what somebody does the moment they
realise what they opened, and it has to bite immediately.

The redundant @jwt_required() above require_permission is gone from 184 routes.
It was belt-and-braces before; now it is actively wrong, because it refuses a
request with no token before the permission check can have an opinion, so a
grant would appear to do nothing.

What a guest may hold is bounded. Any .view or .read permission is grantable.
A WRITE has to be blessed by whoever owns it, through the new
get_guest_grantable_permissions hook (contract 0.21.0), and Secure Send is the
only case that exists: sending a password to a vendor is the one write this
product means to offer somebody who never signs in. Everything else is refused
by the API and greyed out in the grid rather than warned about - a checkbox
that quietly makes deleting an asset anonymous is not a warning anybody reads
twice. A guest write records no creator: the trail says when and from where,
never who, and that is the reason for the bar.

GE-Enforce and Slides gained read permissions so they can be opened to guests
without handing out the manage right. Their read routes accept either through
require_any_permission, so nothing an existing manager could reach has moved.

The navigation endpoint now returns only what the caller can actually reach.
Without that a signed-out visitor gets a sidebar of the whole app where every
link bounces off the login page, which reads as a broken site rather than as a
closed door. A nav item may name several permissions; holding any one is
enough, which is what GE-Enforce and Slides need now that a viewer and a
manager hold different strings.

Two things this found on the way: current_user is a LocalProxy, so
`current_user is None` is False for a request with nobody behind it - harmless
under a required JWT, an AttributeError the moment verification became
optional. And an image pasted into a guide before its first save was never
attached to it, so it was visible to editors and invisible to every reader;
saving now attaches it, and `flask guides fix-images` repairs what is stored.

The guides guest setting written earlier today is gone. Granting guides.view to
the guest role replaces it, which is the point.

docs/PLUGIN-HOOKS.md

@@ -298,6 +298,40 @@ The `Permission` ROWS already in the database are NOT deleted, so roles that
 already reference them keep working until an admin edits the role. A broken
 plugin is isolated in prod and re-raised in dev/test.
 
+### `get_guest_grantable_permissions() -> List[str]`
+
+Names the WRITE permissions this plugin allows a site to grant to the `guest`
+role. Added in contract 0.21.0.
+
+Anyone with no account holds the `guest` role. It is seeded with nothing, and a
+site opens a door by granting it a permission in the role grid - which replaced
+the per-feature booleans (`securesend_allow_guests`, one per kiosk) that used to
+answer the same question one feature at a time.
+
+Any `.view` or `.read` permission is grantable without a plugin saying anything.
+This hook is only for the exception: a write that is genuinely meant to work for
+somebody who never signs in. Naming one here does not open it - it makes the
+checkbox available, and a site still has to tick it.
+
+```python
+class ToolsPlugin(BasePlugin):
+    def get_guest_grantable_permissions(self):
+        # Sending a password to a vendor: the recipient has no account here,
+        # and often neither does the sender. The unguessable token is the
+        # authorisation and the link is one-view.
+        return ['tools.secrets']
+```
+
+The bar for naming one: could this action be attributed to nobody, forever, and
+still be defensible in the audit log? A guest write records no creator - the
+trail says when and from where, never who. Return `[]` (the default) and none of
+this plugin's writes can ever be made anonymous; anything not named is refused
+by the API when an admin tries to grant it, not merely warned about.
+
+Consumed by `guest_grantable_permissions()`, which backs the role API's guard on
+the guest role and the greyed-out checkboxes in the role grid. A plugin may only
+bless permissions that already exist in the catalog, so a typo widens nothing.
+
 ### `get_settings_cards() -> List[Dict]`
 
 Returns settings-catalog card definitions. Added in contract 0.7.0 (ADR-010).