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.

plugins/platforms/photon/README.md

@@ -111,6 +111,7 @@ All env vars are documented in `plugin.yaml`. The most important:
 | `PHOTON_SIDECAR_PORT`     | 8789                       | Loopback port for the sidecar        |
 | `PHOTON_SIDECAR_AUTOSTART`| true                       | Spawn the sidecar on connect         |
 | `PHOTON_DASHBOARD_HOST`   | https://app.photon.codes   | Dashboard API host                   |
+| `PHOTON_SPECTRUM_HOST`    | https://spectrum.photon.codes | Spectrum API host                 |
 | `PHOTON_HOME_CHANNEL`     | your number (set by setup) | Default space for cron delivery — a space id, or a bare E.164 number (resolved to a DM) |
 | `PHOTON_ALLOWED_USERS`    | your number (set by setup) | Comma-separated E.164 allowlist      |
 | `PHOTON_REQUIRE_MENTION`  | false                      | Gate group chats on a wake word      |