move per-user plugin grants to the user's page

Granting was a checklist of every user on each plugin's page, so "what may
this person use?" meant opening every plugin in turn — and the answer lived
on N pages while the connector half of it already lived on one. Both grant
sections now sit together on #users/{id}: same row list, same disabled chip,
same replace-the-whole-set save. The plugin's own page keeps a read-only
roster of who holds it, linking back to each person.

- db: plugin_access::set_for_user, the per-user twin of set_for_user on
  mcp_catalog_access; set_access stays as the inverse read model
- PluginManager: list_grants_for_user / set_grants_for_user, which omit and
  reject manages_own_access plugins (a box that controls nothing is worse
  than no box)
- GET/PUT /api/users/{id}/plugins, mounted next to /users/{id}/connectors;
  PUT /api/plugins/{id}/access is gone, GET remains as the roster

No push after the write, unlike a connector grant: that one gates a runtime
snapshotted at login, while a plugin grant is re-read from plugin_access on
every request that depends on it (sidebar pages, /plugins/mine, and each
inbound channel message), so a revoke lands with no bus event.

Docs updated with where access is granted, and why mobile-connector is
absent from that list.

web/i18n/en.js

@@ -892,9 +892,9 @@ export default {
   'plugin_page.loading':       'Loading…',
   'plugin_page.unavailable':   'This page is not available (plugin disabled or page not granted).',
   'plugins.badge.user_page':   'user page',
-  'plugins.access.desc':       'Tick a box to let that person see and configure this plugin. Saving replaces the whole list.',
-  'plugins.access.empty':      'No users.',
-  'plugins.access.save':       'Save access',
+  'plugins.access.desc':       'Who can see and use this plugin. Access is granted on each person\'s own page, next to their connectors. Admins always have access.',
+  'plugins.access.nobody':     'Nobody has been granted this plugin yet.',
+  'plugins.access.manage':     'Manage access from Users',
   'plugins.error.required':    '"{field}" is required.',
   'plugins.catalog.configure': 'Configure',
   'plugins.health.ok':           'active',
@@ -1065,6 +1065,7 @@ export default {
   'users.detail.back':                 'Users',
   'users.detail.profile':              'Profile',
   'users.detail.connectors':           'Connectors',
+  'users.detail.plugins':              'Plugins',
   'users.detail.security':             'Security',
   'users.detail.saved':                'Saved',
   'users.detail.delete_hint':          'Deletes this user, their database and all conversation history.',
@@ -1077,6 +1078,10 @@ export default {
   'users.conn.empty':                  'No connectors registered yet.',
   'users.conn.disabled':               'disabled',
 
+  'users.plug.hint':                   'Tick a plugin to let this user see and use it. A disabled plugin can be granted now and will appear once you enable it.',
+  'users.plug.empty':                  'No plugins available.',
+  'users.plug.admin_note':             'Admins can use every enabled plugin, whatever is ticked here.',
+
   'users.modal.create_title':          'New user',
   'users.modal.username':              'Username',
   'users.modal.display_name':          'Display name',