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/api/router.go

@@ -160,6 +160,11 @@ func NewRouter(db *sql.DB, notify NotifyConfig, cfg config.Config) http.Handler
 		r.Post("/api/teams/{teamID}/members", handleAddTeamMember(db))
 		r.Delete("/api/teams/{teamID}/members/{userID}", handleRemoveTeamMember(db))
 
+		// A team's own OIDC group binding: which provider groups grant member
+		// and owner access to it.
+		r.Get("/api/teams/{teamID}/oidc-groups", handleGetTeamOIDCGroups(db))
+		r.Put("/api/teams/{teamID}/oidc-groups", handleSetTeamOIDCGroups(db))
+
 		// Invite links into this team.
 		r.Get("/api/teams/{teamID}/invites", handleListInvites(db))
 		r.Post("/api/teams/{teamID}/invites", handleCreateInvite(db, notify.PublicURL))