Goujana subscriptions

Identity + billing lifecycle of ONE active Goujana add-on plan for ONE account — decoupled from what that plan actually grants (see GoujanaPayrollLimits below). One row per (account, product_line): an account can hold a payroll plan AND a future industry plan at the same time, each its own row here. Owner's explicit call (2026-08-14): Goujana add-on plans (nómina, future industries) are a SEPARATE product line from the general ERP plans (goujana_gateway.plans) — "no compiten entre ellos" — and their capacity data must live entirely inside goujana_gateway (this app, this tenant's own database), never as new fields on the shared core database_router_s.UserDb/SubscriptionType tables that every Sofisis tenant migrates. This is also why this model is NOT layered onto GoujanaLeadFollowUp above: that model's cardinality (one row per lead, `unique=True`) and lifecycle (funnel stage that goes stale once `won`) don't fit a subscription that must support multiple concurrent product lines per account and keeps mutating for years after the sale — same no-FK `user_db_id` convention as that model, not the same table. See README_ESTRUCTURA_SUSCRIPCIONES_GOUJANA.md for the full rationale and the migration this replaced (GoujanaPayrollSubscription, one single table mixing lifecycle + payroll-specific fields — replaced with zero real rows in any environment, so no data migration was needed).