feat(tenants): TenantStatus enum + status endpoint + optional mode

[jlc488]

Fourth PR in the admin-ui ↔ backend contract-gap series (after #14 menus, #15 audit-logs, #16 policies). The Tenants page needs a three-state lifecycle (ACTIVE / SUSPENDED / ARCHIVED) and a single setStatus endpoint, not the boolean active flag plus two separate /activate / /deactivate routes; it also doesn't send mode when provisioning a tenant.

Summary

Status enum

• New TenantStatus enum in tenant-api: ACTIVE / SUSPENDED / ARCHIVED. Vocabulary matches the UI Tag set so wire shape and presentation line up without translation.

Schema (V10) — online-safe rewrite of platform_tenant

1. ADD COLUMN status VARCHAR(16) (nullable).
2. UPDATE back-fill: active = true → 'ACTIVE', active = false → 'SUSPENDED' (treat as "paused", not "soft-deleted").
3. ALTER COLUMN status SET NOT NULL, swap idx_platform_tenant_active → idx_platform_tenant_status, DROP COLUMN active.

Entity / metadata

• PlatformTenantEntity: active boolean → status TenantStatus (@Enumerated(STRING)). All-args constructor updated.
• TenantMetadata: active boolean → status TenantStatus, with a null-check in the canonical constructor.

Service

• TenantService gains setStatus(id, status) as the primary lifecycle method.
• activate(id) / deactivate(id) demoted to default methods that dispatch through setStatus(ACTIVE) / setStatus(SUSPENDED). All prior callers keep working unchanged.
• DefaultTenantService.create now persists status = ACTIVE on new tenants.

Controller

• POST /tenants: mode is now optional (defaults to TenantMode.SINGLE when the UI doesn't send it).
• PUT /tenants/{id}/status (new): the admin UI's single setter; body is { "status": "ACTIVE" | "SUSPENDED" | "ARCHIVED" }.
• PUT /tenants/{id}/activate and /deactivate kept as backward-compatible shortcuts that route through setStatus.

Test plan

• ./gradlew build green (55 tasks, all sample-app + tenant-core tests pass; migration runs cleanly against Testcontainers Postgres)
• Manual: provision a tenant via the admin UI without mode, change status to SUSPENDED, archive

Closes the series

This was the last large gap. The Diagnostics + Settings field-name mismatches (cosmetic JSON-key renames) can land as a small follow-up PR or be absorbed on the admin-ui side, depending on which direction you prefer.