fix: validate roles, guard last owner, scope bootstrap status

Security review of e70b2f0. The UI gating was correctly backed by
RequireRole everywhere; these are the missing validation gaps behind it.

- UpdateUserRole and createOrgUser accepted any role string verbatim, so
  an admin could self-promote to owner, create an owner outright, or set
  a junk role that silently stripped a user's access. Roles are now
  whitelisted, only an owner may grant or remove the owner role, and an
  actor cannot change their own.
- Neither demote nor delete guarded the last owner, so an org could reach
  zero owners. Both now refuse when no owner would remain, returning 409.
  Self-delete rejected.
- CountUsers counted across all orgs, so a locked-out org could never
  re-bootstrap once another tenant existed, and the unauthenticated
  bootstrap-status endpoint reported instance-wide state. It now answers
  per-org on an org host, falling back to global only on the apex.
- HandleMe repeats the middleware's host/org check; it sits outside the
  middleware so it can still return its own 401.
- Post-bootstrap now sends the new owner to their org host's login page.
  The session cookie is deliberately scoped to the exact host, so the old
  redirect landed them unauthenticated.
- AuthProvider renders an error state instead of mounting the shell with
  a null user when /auth/me fails for a reason other than 401.
- api.ts unwraps {"error": ...} so these messages render as text.

server/internal/models/user.go

@@ -6,6 +6,22 @@ import (
 	"go.mongodb.org/mongo-driver/v2/bson"
 )
 
+// Org membership roles. These are the only values ever written to User.Role;
+// anything arriving from a client must be checked with ValidRole first.
+const (
+	RoleOwner  = "owner"
+	RoleAdmin  = "admin"
+	RoleMember = "member"
+)
+
+func ValidRole(role string) bool {
+	switch role {
+	case RoleOwner, RoleAdmin, RoleMember:
+		return true
+	}
+	return false
+}
+
 type User struct {
 	ID           bson.ObjectID `bson:"_id,omitempty" json:"_id,omitempty"`
 	UserID       string        `bson:"user_id" json:"user_id"`