docs: add enterprise deployment guide

[jeroenrinzema]

Adds a single, dense operations page for running Lunogram in an enterprise setting, published publicly and marked enterprise-only with the existing <EnterpriseFeature> banner.

What it covers

• What Enterprise adds — broadcasts, team invites, sending domains, block editor, AI template builder, and the fact that this is a build-tag distribution rather than a licence key.
• Architecture — the four components and which of them are stateless, with a diagram.
• Requirements — dependency versions, the four PostgreSQL databases and what each holds, the per-replica connection budget, and that OpenFGA runs in-process.
• Deploying — a minimal Kubernetes manifest, and why probing GET / is a meaningful readiness signal given there is no dedicated health endpoint.
• Configuration — the full environment-variable reference, grouped, taken from the config structs.
• Multi-replica behaviour — leader election, consumer distribution, and the three things that must be shared across replicas (AUTH_CONSOLE_SIGNING_KEY, CSRF_SECRET, object storage).
• Hardening, upgrades and backups — including the -migrate flag and DATABASE_MIGRATE=false for controlled rolling upgrades.

Notes

Two defaults are called out as warnings because they are quietly wrong in a real deployment: RATE_LIMIT_TRUSTED_PROXY_HOPS=0 puts every unauthenticated request behind a load balancer into one bucket, and a per-boot random CSRF_SECRET breaks across replicas.

The page is added to the sidebar under a new Deployment group between Overview and Guides. Media storage keeps its own page and is linked rather than duplicated.

pnpm build in docs/ passes; the page was rendered locally to confirm the diagram and the enterprise banner.