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/plugin.yaml

@@ -46,6 +46,10 @@ optional_env:
     description: "Photon Dashboard API host (default https://app.photon.codes)"
     prompt: "Dashboard host"
     password: false
+  - name: PHOTON_SPECTRUM_HOST
+    description: "Photon Spectrum API host (default https://spectrum.photon.codes)"
+    prompt: "Spectrum API host"
+    password: false
   - name: PHOTON_ALLOWED_USERS
     description: "Comma-separated E.164 phone numbers allowed to talk to the bot"
     prompt: "Allowed users (comma-separated)"