Phase 4: SSO scaffolding, RBAC enforcement, tenant-scoped dashboards, audit logging, K8s deployment

RBAC (api/internal/authz) is live on /query and /dashboards, backed by a
new enterprise/ module (session issuance, audit logging, RBAC storage,
OIDC/SAML protocol wiring) that core never imports -- only calls over
HTTP. Found and fixed a real cross-tenant vulnerability in dashboards
(no tenant_id filtering at all) while writing the threat model doc.

Two things are explicitly NOT done, documented rather than hidden:
tenant isolation for log data itself (/query still shares one ClickHouse
connection and Tantivy index across every tenant -- RBAC controls who
can query, not what a query can see), and human SSO login (protocol
wiring exists, no HTTP handler calls it yet). See
docs/security/threat-model.md and docs/phase-4-runbook.md.

Also adds deploy/ (Go Operator + Helm chart, validated offline only --
no cluster was reachable in this environment).

enterprise/internal/tenant/tenant_test.go

@@ -0,0 +1,37 @@
+package tenant
+
+import (
+	"context"
+	"testing"
+)
+
+func TestFromContextRoundTrip(t *testing.T) {
+	id := TrustFromValidatedSession("acme-corp")
+	ctx := WithContext(context.Background(), id)
+
+	got, ok := FromContext(ctx)
+	if !ok {
+		t.Fatalf("expected FromContext to find a tenant")
+	}
+	if got.String() != "acme-corp" {
+		t.Fatalf("got %q, want %q", got.String(), "acme-corp")
+	}
+}
+
+func TestFromContextMissing(t *testing.T) {
+	_, ok := FromContext(context.Background())
+	if ok {
+		t.Fatalf("expected no tenant in a bare context")
+	}
+}
+
+func TestFromContextDoesNotMatchUnrelatedStringKey(t *testing.T) {
+	// The unexported contextKey type is what closes the "collision" gap
+	// the design doc calls out -- a context.WithValue using a plain
+	// string key must not be found by FromContext.
+	ctx := context.WithValue(context.Background(), "tenant_id", "spoofed") //nolint:staticcheck
+	_, ok := FromContext(ctx)
+	if ok {
+		t.Fatalf("FromContext must not find a value set under an unrelated key type")
+	}
+}