TerdutTeam: mint and surface a real invite link (spec.invite)

The actual fix for the human-onboarding gap niklas/terdut-server#23 found --
not a terdut-server change at all. A team-scoped credential is already
owner-equivalent for POST/GET/DELETE /api/teams/{teamID}/invites
(requireTeamOwner's synthetic-membership mechanism, ratified not
accidental per that repo's SERVICE-ACCOUNTS.md), and invite redemption
bypasses signup_mode entirely -- this TerdutTeam controller just never
grew a feature to use either fact.

New spec.invite{enabled, role (member|owner, default member), maxUses
(1-100, default 1)} and status.inviteSecretRef. The Secret lives in the
TerdutTeam's OWN namespace, not the operator's: unlike
status.credentialsSecretRef (a durable, high-privilege credential, kept
operator-side per DESIGN.md §6), an invite is bounded and limited-use,
meant for this namespace's own human operators to read and hand out --
same precedent as TerdutAlertSource's status.webhookURLSecretRef, same-
namespace and OwnerReference'd so deleting the TerdutTeam garbage-collects
it automatically.

internal/controller/terdutteam_invite.go: mints on first spec.invite.enabled,
refreshes a day ahead of terdut-server's fixed 7-day TTL (reading the
Secret's own stored expiresAt, no extra server round-trip per reconcile),
revokes server-side and deletes the Secret when flipped back to false. A
lost invite Secret is silently re-minted rather than treated as
unrecoverable the way TerdutAlertSource's webhook key is -- nothing
external holds a durable dependency on one specific invite link staying
stable, it's read once by one human and handed out.

New tdclient.Invite/CreateInvite/RevokeInvite. New envtest coverage: mint
into the team's own namespace, refresh-before-expiry, revoke-on-disable
(internal/controller/terdutteam_controller_test.go's new "spec.invite"
Describe block), plus the fake server growing invite support
(terdutserver_controller_test.go) -- its handleTeamSubPath dispatcher was
split further (deadman switches into their own handleDeadmanSubPath,
matching the existing handleIntegrationSubPath precedent) to stay under
golangci-lint's gocyclo threshold with the new route added.

examples/demo updated to prove this end to end: 02-team-platform.yaml
turns on spec.invite; run-demo.sh's bootstrap_login/join_demo_teams (the
psql signup_mode flip + a direct team_members INSERT) are replaced by
redeem_platform_invite (reads status.inviteSecretRef, a real POST
/api/signup with the invite token) and join_payments_team (POST
/api/teams/{teamID}/members using Payments' own credential and alice's
user id resolved via GET /api/users, deliberately not given its own
spec.invite, so the demo shows both onboarding paths this feature
unlocks) -- zero kubectl exec/psql calls remain anywhere in the script.
README.md's "First login" section rewritten to match; it no longer
documents the admin-token curl call that 403s against current
terdut-server (niklas/terdut-server#23).

Depends on niklas/terdut-server#24 (the callerMayManageServiceAccount fix
for terdut-operator#3) being released before this is deployed for real --
not required to build or test this change itself, since the envtest fake
never modeled that authorization gap to begin with.

internal/controller/terdutteam_controller.go

@@ -131,6 +131,9 @@ func (r *TerdutTeamReconciler) Reconcile(ctx context.Context, req ctrl.Request)
 	if err := teamClient.SetTeamOIDCGroups(ctx, team.Status.TeamID, team.Spec.OIDC.MemberGroup, team.Spec.OIDC.OwnerGroup); err != nil {
 		return ctrl.Result{}, fmt.Errorf("PUT /api/teams/%d/oidc-groups: %w", team.Status.TeamID, err)
 	}
+	if err := r.reconcileInvite(ctx, &team, teamClient); err != nil {
+		return ctrl.Result{}, err
+	}
 
 	meta.SetStatusCondition(&team.Status.Conditions, metav1.Condition{
 		Type:    terdutv1alpha1.ConditionReady,