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).

metadata/migrations/0019_seed_default_tenant.sql

@@ -0,0 +1,8 @@
+-- Every Phase 0-3 row (dashboards, alert_rules, notification_targets --
+-- see their tenant_id DEFAULT 'default' columns) belongs to this tenant.
+-- Marked 'active' immediately: this data already exists and is already
+-- being served, unlike a genuinely new tenant that must pass through
+-- provisioning first.
+INSERT INTO tenants (id, display_name, status)
+VALUES ('default', 'Default', 'active')
+ON CONFLICT (id) DO NOTHING