feat(auth): membership authority endpoint + fail-closed API auth (RBAC Phase 1)

GET /v1/users/{id}/memberships answers 'which tenants does this JWT
subject belong to, with which org_roles and entitlements' (ratified
auth design, model B2). Keycloak supplies user->tenant links and roles
via the Adapter (attribute projection until the realm migrates to
Organizations); registered tenants override status/plan/products from
registry tables.

New internal/authn verifies Keycloak bearer tokens (OIDC discovery +
JWKS, audience AUTH_EXPECTED_AUDIENCE, default tenant-registry). With
AUTH_ENABLED=true all routes except /healthz + /readyz require a token
and the server refuses to start if the verifier cannot initialize;
false (dev default) keeps the API open. Completes the M5.2-deferred
org_roles lookup and replaces the 'M4.3 adds JWT validation' TODO in
the spec.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/keycloak/users.go

@@ -0,0 +1,74 @@
+package keycloak
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"strings"
+)
+
+// userRepresentation is the slice of the Admin API's UserRepresentation we
+// need. Attributes arrive as map[name][]values; the KC admin console writes
+// multivalued attributes either as separate list entries or as one entry
+// joined with "##", so both shapes must be accepted.
+type userRepresentation struct {
+	ID         string              `json:"id"`
+	Username   string              `json:"username"`
+	Enabled    bool                `json:"enabled"`
+	Attributes map[string][]string `json:"attributes"`
+}
+
+// Memberships implements Adapter against GET /admin/realms/{realm}/users/{id}.
+func (a *HTTPAdapter) Memberships(ctx context.Context, userID string) ([]Claims, error) {
+	var u userRepresentation
+	resp, err := a.adminCall(ctx, http.MethodGet, "/users/"+userID, nil, &u)
+	if err != nil {
+		return nil, err
+	}
+	if resp.StatusCode == http.StatusNotFound {
+		_ = resp.Body.Close()
+		return nil, ErrUserNotFound
+	}
+	if resp.StatusCode/100 != 2 {
+		_ = resp.Body.Close()
+		return nil, fmt.Errorf("keycloak get user: %d", resp.StatusCode)
+	}
+	return claimsFromAttributes(u.Attributes), nil
+}
+
+// claimsFromAttributes builds the membership list from a user's attribute
+// projection. A user with no tenant_id and no tenant_slug attribute simply
+// has no memberships — that is a valid state, not an error.
+func claimsFromAttributes(attrs map[string][]string) []Claims {
+	c := Claims{
+		TenantID:     attrValue(attrs, "tenant_id"),
+		TenantSlug:   attrValue(attrs, "tenant_slug"),
+		OrgRoles:     attrValues(attrs, "org_roles"),
+		Products:     attrValues(attrs, "products"),
+		Plan:         attrValue(attrs, "plan"),
+		TenantStatus: attrValue(attrs, "tenant_status"),
+	}
+	if c.TenantID == "" && c.TenantSlug == "" {
+		return []Claims{}
+	}
+	return []Claims{c}
+}
+
+func attrValue(attrs map[string][]string, key string) string {
+	if vs := attrValues(attrs, key); len(vs) > 0 {
+		return vs[0]
+	}
+	return ""
+}
+
+func attrValues(attrs map[string][]string, key string) []string {
+	out := []string{}
+	for _, entry := range attrs[key] {
+		for _, v := range strings.Split(entry, "##") {
+			if v = strings.TrimSpace(v); v != "" {
+				out = append(out, v)
+			}
+		}
+	}
+	return out
+}