docs: add Phase 6 multi-tenancy plan to roadmap

- Row-level isolation architecture
- Per-tenant Entra + Graph credentials
- License-gated premium feature
- Deferred until SIEM export and alerting are production-tested

ROADMAP.md

@@ -72,3 +72,32 @@ Goal: add AI-powered analysis and external tool integration.
 ## Completed in this PR
 All Phase 5 items marked done were implemented in v1.3.0–v1.5.0.
 Redis caching + async queue implemented in v1.6.0, switched to Valkey.
+UI polish (topbar, footer, clickable pills) in v1.6.1–v1.6.4.
+
+---
+
+## Phase 6: Multi-Tenancy (Premium) ⏸️
+Goal: allow MSPs to manage multiple client tenants from a single deployment.
+
+Status: **Planned — not started**. Architecture designed, pending validation of core features (SIEM export, alerting) in production first.
+
+### Architecture
+- Row-level isolation: `tenant_id` field on every MongoDB document
+- Each tenant has their own Microsoft Entra tenant + app registration credentials
+- Auth: user's JWT `tid` claim maps to tenant config automatically
+- Super-admin role for MSP staff to access all tenants
+
+### Implementation phases
+- **Phase 6.1** (2–3 days): Tenant model & registry, tenant-aware data layer, per-tenant Graph API auth
+- **Phase 6.2** (1 day): Tenant-scoped API routes, tenant-specific config endpoints
+- **Phase 6.3** (2 days): Frontend tenant switcher, tenant name display, admin page
+- **Phase 6.4** (1 day): License gating — signed JWT `LICENSE_KEY` gates multi-tenant mode
+
+### Licensing model
+- Single-tenant: remains MIT/free
+- Multi-tenant: premium feature requiring a signed license key
+- License key is a JWT with claims: `plan`, `max_tenants`, `exp`, `features`
+- Offline license generation tool included
+
+### Effort estimate
+~7–9 days total. Deferred until SIEM export and alerting are battle-tested.