Let each team name its own OIDC group, not a global mapping

Team membership from single sign-on used to come from one env var,
TERDUT_OIDC_GROUP_MAPPINGS, matched against a team by name and creating
the team if none existed. That put the decision in the server's
environment rather than the team's own hands, needed a restart to
change, and let a typo in a team name silently create a stray team.

Each team now carries its own oidc_member_group and oidc_owner_group,
set by its owner (or an administrator) from the Members tab, or PUT
/api/teams/{teamID}/oidc-groups. The "highest role wins" rule
TERDUT_OIDC_GROUP_MAPPINGS used to apply across mappings now applies
across one team's own two fields: being in both makes somebody an
owner. The sync no longer creates a team by name; a group only ever
grants into a team that already exists.

This is a breaking change for anyone already using
TERDUT_OIDC_GROUP_MAPPINGS, deliberately not auto-migrated: an
OIDC-sourced membership is dropped at a user's next sign-in until its
team's owner re-sets the group. The README's OIDC section spells out
the migration and the risk of a visible access gap during it.

TERDUT_OIDC_ADMIN_GROUP and TERDUT_OIDC_ALLOWED_GROUPS are untouched --
only team membership moved. terdut-tui needs no change: it only reads
GET /api/teams and GET /api/teams/{id}/members, and neither response
shape moved.

internal/web/static/js/team.js

@@ -101,8 +101,12 @@ async function load(id) {
     return { members, schedule };
   }
   if (tab === 'members') {
-    const [members, users] = await Promise.all([api.teamMembers(id), allUsers()]);
-    return { members, users };
+    const [members, users, oidcGroups] = await Promise.all([
+      api.teamMembers(id),
+      allUsers(),
+      state.auth?.oidc?.enabled ? api.oidcGroups(id) : null,
+    ]);
+    return { members, users, oidcGroups };
   }
   if (tab === 'escalation') {
     const [members, escalation] = await Promise.all([api.teamMembers(id), api.escalation(id)]);
@@ -1046,6 +1050,73 @@ function shiftCell(m) {
     : h('span', { text: 'not scheduled' });
 }
 
+// The team's own OIDC group binding, shown only on an SSO-enabled install:
+// which group grants membership and which grants ownership. Read-only text
+// for a member, an edit sheet for an owner — the server enforces the same
+// split on the endpoint underneath.
+function oidcGroupsCard() {
+  if (!state.auth?.oidc?.enabled) return null;
+  const g = data.oidcGroups || { member_group: '', owner_group: '' };
+  return h('div', { class: 'card' },
+    h('div', { class: 'card-head' },
+      h('h2', { text: 'Single sign-on' }),
+      isOwner() && h('button', {
+        class: 'btn', type: 'button', text: 'Edit', onclick: openOidcGroupsEditor,
+      })),
+    h('p', { class: 'muted small' },
+      'Members of the group below are added to this team automatically at ',
+      'sign-in; members of the owner group become owners. Leave a field ',
+      'blank to grant nothing this way.'),
+    h('dl', { class: 'user-facts' },
+      fact('Member group', g.member_group || '—'),
+      fact('Owner group', g.owner_group || '—'),
+    ),
+  );
+}
+
+function fact(label, value) {
+  return [h('dt', { text: label }), h('dd', { text: value })];
+}
+
+function openOidcGroupsEditor() {
+  const g = data.oidcGroups || { member_group: '', owner_group: '' };
+  const memberGroup = h('input', {
+    type: 'text', value: g.member_group, placeholder: 'e.g. sre', autofocus: true,
+  });
+  const ownerGroup = h('input', { type: 'text', value: g.owner_group, placeholder: 'e.g. sre-leads' });
+  const problem = h('p', { class: 'load-error', hidden: true });
+
+  const form = h('form', { class: 'stacked-form' },
+    h('label', {}, 'Member group ', memberGroup),
+    h('label', {}, 'Owner group ', ownerGroup),
+    h('p', { class: 'muted small' },
+      'A person in both becomes an owner. Whoever the group lists is kept in ',
+      'sync at their next sign-in — a member added by hand can still be made ',
+      'an owner, but not the other way round.'),
+    problem,
+    h('div', { class: 'sheet-actions' },
+      h('button', { class: 'btn', type: 'button', text: 'Cancel', onclick: () => closeSheet(false) }),
+      h('button', { class: 'btn btn-primary', type: 'submit', text: 'Save' })));
+
+  form.addEventListener('submit', async (e) => {
+    e.preventDefault();
+    try {
+      await api.setOidcGroups(teamID, {
+        member_group: memberGroup.value.trim(),
+        owner_group: ownerGroup.value.trim(),
+      });
+    } catch (err) {
+      problem.textContent = err.message;
+      problem.hidden = false;
+      return;
+    }
+    closeSheet(true);
+    refresh();
+  });
+
+  openSheet(() => [h('h2', { class: 'sheet-title', text: 'Single sign-on groups' }), form]);
+}
+
 function membersCard() {
   const members = data.members || [];
   const owners = members.filter((m) => m.role === 'owner').length;
@@ -1090,7 +1161,7 @@ function membersCard() {
     );
   });
 
-  return h('div', { class: 'card' },
+  return [oidcGroupsCard(), h('div', { class: 'card' },
     h('div', { class: 'card-head' },
       h('h2', { text: 'Members' }),
       isOwner() && h('button', {
@@ -1108,7 +1179,7 @@ function membersCard() {
             h('th'))),
           h('tbody', {}, rows)))
       : h('p', { class: 'muted', text: 'Nobody is in this team.' }),
-  );
+  )];
 }
 
 // One sheet for both jobs a member's row has: who, and as what. Adding is