plugins: merge the user Plugins page into per-plugin sidebar pages

The generic per-user #plugins page is gone: a plugin with per-user
settings hosts them in its own web_pages() sidebar page instead
(Telegram's pairing page is new; Honcho's opt-in page already existed).
The admin catalog moves from #plugin-catalog to #plugins (old hash
redirected), and user_config_schema is removed from the Plugin trait,
the API DTOs and both plugins — the my-config endpoint, the
plugin_user_configs store and the update_user_config hook stay, now
driven by each plugin's own page fragment.

crates/skald-core/src/db/plugin_user_configs.rs

@@ -1,10 +1,11 @@
 //! Per-user plugin configuration blobs (`plugin_user_configs` table).
 //!
 //! Registry table in `system.db` — **admin-readable, never secrets**. A plugin
-//! with a non-empty `user_config_schema()` lets each granted user submit their
-//! own settings from the UI (e.g. Telegram's pairing code); the plugin's
-//! `update_user_config` hook validates and stores here. `plugin_id` is a bare
-//! TEXT for the same reason as `plugin_access`.
+//! with per-user settings surfaces them in its own `web_pages()` fragment
+//! (e.g. Telegram's pairing page); the submission travels through the core
+//! `PUT /api/plugins/{id}/my-config` endpoint into the plugin's
+//! `update_user_config` hook, which validates and stores here. `plugin_id` is
+//! a bare TEXT for the same reason as `plugin_access`.
 
 use anyhow::Result;
 use serde_json::Value;