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.

docs/plugins/honcho.md

@@ -26,7 +26,7 @@ Streams a user's completed chat turns to an external [Honcho](https://honcho.dev
 
 ## Per-user setup
 
-Long-term memory is **off for every user until they turn it on themselves**. Once the plugin is enabled and the user has been granted access, they'll see a **"Long-term memory"** page in their sidebar with a single opt-in toggle. If a user asks the assistant to "remember things long-term" or asks why it doesn't remember past conversations, and this plugin is enabled, point them to that page rather than trying to enable it on their behalf.
+Long-term memory is **off for every user until they turn it on themselves**. Once the plugin is enabled and the user has been granted access (admin: Users → that person → **Plugins** → tick Honcho), they'll see a **"Long-term memory"** page in their sidebar with a single opt-in toggle. If a user asks the assistant to "remember things long-term" or asks why it doesn't remember past conversations, and this plugin is enabled, point them to that page rather than trying to enable it on their behalf.
 
 ## Notes