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/0026_seed_default_tenant_data_source.sql

@@ -0,0 +1,7 @@
+-- The 'default' tenant's single data source, pointing at the one
+-- ClickHouse database ("sentry") and Tantivy index every Phase 0-3
+-- deployment already uses -- see api/internal/config's CLICKHOUSE_DATABASE
+-- default and search's index path default.
+INSERT INTO data_sources (id, tenant_id, name, clickhouse_database_name, tantivy_index_path)
+SELECT '00000000-0000-0000-0000-000000000001', 'default', 'default', 'sentry', '/var/lib/sentry-search'
+WHERE NOT EXISTS (SELECT 1 FROM data_sources WHERE tenant_id = 'default')