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).

deploy/helm/sentry/templates/tenants.yaml

@@ -0,0 +1,14 @@
+{{- if .Values.enterprise.enabled }}
+{{- range .Values.tenants }}
+---
+apiVersion: sentry.io/v1alpha1
+kind: Tenant
+metadata:
+  name: {{ .name }}
+  labels:
+    {{- include "sentry.labels" $ | nindent 4 }}
+spec:
+  displayName: {{ .displayName | default .name | quote }}
+  suspended: {{ .suspended | default false }}
+{{- end }}
+{{- end }}