fix(photon): migrate user API calls to Spectrum backend

Switch `list_users`, `find_user_by_phone`, `create_user`,
`register_user_if_absent`, and `refresh_user_numbers` from the
Dashboard API (Bearer token) to the Spectrum API (Basic auth with
project credentials). Update response unwrapping to handle the nested
`data.users` envelope returned by Spectrum, add `_spectrum_host()`
resolver, `_basic()` header helper, and structured error helpers.
Update tests, docs, and plugin.yaml accordingly.

website/docs/user-guide/messaging/photon.md

@@ -222,6 +222,7 @@ Common issues:
 | `PHOTON_REQUIRE_MENTION`  | `false`            | Require a wake word before responding in groups |
 | `PHOTON_MENTION_PATTERNS` | Hermes wake words  | JSON list / comma / newline regex patterns for group mentions |
 | `PHOTON_DASHBOARD_HOST`   | `app.photon.codes` | Override the dashboard / device-login host |
+| `PHOTON_SPECTRUM_HOST`    | `spectrum.photon.codes` | Override the Spectrum API host |
 
 [photon]: https://photon.codes/
 [app]: https://app.photon.codes/