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/Chart.yaml

@@ -0,0 +1,12 @@
+apiVersion: v2
+name: sentry
+description: >-
+  Sentry: Kubernetes-native distributed log aggregation. Phase 4 adds
+  multi-tenant-aware deployment (per-tenant ClickHouse credential Secrets
+  via the tenant-operator, optional enterprise-auth) on top of the same
+  single-cluster topology Phases 0-3 ran under docker-compose -- see
+  deploy/README.md for what "multi-tenant-aware" does and doesn't mean
+  here.
+type: application
+version: 0.4.0
+appVersion: "phase-4"