Memberships: groups kept by portal itself, a person by phone number first

A membership is a case in the built-in portal_memberships bucket:
invited by a desk, a grants, the seed or the responsible pass; active
from the person's first sign-in, when the provider's subject, phone
number or address matches the record; removed by a desk, and invited
again after that. One record per group and person however it was
keyed, and at sign-in every record that matched learns what the
provider verified. The active memberships join the groups claim, so
Cedar sees the same parents as before.

PEOPLE_BACKEND picks Kanidm or memberships; unset, a KANIDM_API_TOKEN
means Kanidm. The NATS access service looks a person's groups up when
a question names them by sub, phone or email and lists none. The
sign-in asks for the phoneNumber scope and reads it from userinfo.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GT37Z1Xtfd9pUuQtMTg6Yt

src/auth.rs

@@ -14,6 +14,10 @@ pub struct User {
     /// live, so a group change only takes effect on the next login.
     #[serde(default)]
     pub groups: Vec<String>,
+    /// The phone number the provider verified (`phone_number`, from
+    /// userinfo), when it gives one: what a membership is keyed by first.
+    #[serde(default)]
+    pub phone: Option<String>,
     /// The address Kanidm holds for them (the `email` claim), so a page
     /// can know it is talking to the person it names as responsible.
     #[serde(default)]