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/plugin-telegram-bot/Cargo.toml

@@ -7,6 +7,7 @@ edition = "2024"
 core-api    = { path = "../core-api" }
 anyhow      = "1"
 async-trait = "0.1"
+axum        = { version = "0.8" }
 serde       = { version = "1", features = ["derive"] }
 serde_json  = "1"
 tokio       = { version = "1", features = ["full"] }