feat(tenants): add multi-tenant module — tenant CRUD, members, invites
- types.ts: TenantDoc, TenantMemberDoc, TenantInviteDoc + 4 Zod schemas
- repository.ts: tenant CRUD, member CRUD, invite lifecycle, slug uniqueness
- routes.ts: 10 endpoints (tenant CRUD, invite, accept, member role, remove)
- tenants.test.ts: 16 schema + plan-limits tests
- Plan limits: free(5), starter(25), pro(100), enterprise(10k) members
- SHA-256 hashed invite tokens with 7-day expiry
- Cosmos containers: tenants, tenant_members, tenant_invites