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/api.js

@@ -135,6 +135,10 @@ export const addTeamMember = (id, userID, role) =>
   call('POST', `/teams/${id}/members`, { body: { user_id: userID, role } });
 export const removeTeamMember = (id, userID) => call('DELETE', `/teams/${id}/members/${userID}`);
 
+// Which OIDC groups grant member and owner access to this team.
+export const oidcGroups = (id) => call('GET', `/teams/${id}/oidc-groups`);
+export const setOidcGroups = (id, body) => call('PUT', `/teams/${id}/oidc-groups`, { body });
+
 export const integrations = (id) => call('GET', `/teams/${id}/integrations`);
 export const createIntegration = (id, name) =>
   call('POST', `/teams/${id}/integrations`, { body: { name } });