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/0024_create_dashboard_permissions.sql

@@ -0,0 +1,14 @@
+-- Additive-only per-resource grant: lets a specific user exceed their
+-- tenant-baseline role on one dashboard (e.g. an Editor granted Admin on
+-- a dashboard they don't own). No deny-overrides -- named non-goal in
+-- /docs/phase-4-rbac-design.md and CLAUDE.md's Phase 4 exit criteria.
+CREATE TABLE IF NOT EXISTS dashboard_permissions
+(
+    id           UUID PRIMARY KEY,
+    dashboard_id UUID NOT NULL REFERENCES dashboards(id) ON DELETE CASCADE,
+    user_id      UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    role         TEXT NOT NULL CHECK (role IN ('viewer', 'editor', 'admin')),
+    granted_by   UUID REFERENCES users(id),
+    created_at   TIMESTAMPTZ NOT NULL DEFAULT now(),
+    UNIQUE (dashboard_id, user_id)
+)