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/Dockerfile

@@ -0,0 +1,12 @@
+# Commercial-license module, built the same way as every other Go
+# service here -- no /proto dependency, context is enterprise/ itself,
+# same shape as cli/Dockerfile and alerting/Dockerfile.
+#   docker build -f enterprise/Dockerfile -t sentry-enterprise-auth enterprise/
+FROM golang:1.25-alpine AS builder
+WORKDIR /src
+COPY . .
+RUN CGO_ENABLED=0 GOOS=linux go build -o /out/enterprise-auth ./cmd/enterprise-auth
+
+FROM gcr.io/distroless/static-debian12
+COPY --from=builder /out/enterprise-auth /enterprise-auth
+ENTRYPOINT ["/enterprise-auth"]