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.

docs/plugins/orpheus_tts_3b.md

@@ -19,7 +19,7 @@ The model is gated on HuggingFace, so it requires a personal access token before
 ## Enabling & configuring (admin)
 
 1. Get a HuggingFace token and store it as the secret `HUGGINGFACE_TOKEN` (see above).
-2. Plugin catalog → **Orpheus TTS 3B** → enable, then **Configure**.
+2. Plugins page → **Orpheus TTS 3B** → enable, then **Configure**.
 3. Fields:
    - **`quantization`** (`none` | `int8` | `int4`, default `int8`) — lower precision uses less VRAM at some quality cost.
    - **`voice`** (`tara` | `dan` | `leah` | `zac` | `zoe` | `mia` | `julia` | `leo`, default `tara`).