docs: add SaaS transformation implementation plan
28 tasks across 8 chunks:
- Chunk 1: Core infrastructure (DB-per-tenant, env, JWT, pools)
- Chunk 2: FIEL dual storage + encryption fix
- Chunk 3: Email service (Nodemailer + Gmail SMTP)
- Chunk 4: MercadoPago payments (subscriptions, webhooks)
- Chunk 5: Plan enforcement (limits, feature gates)
- Chunk 6: Tenant provisioning integration
- Chunk 7: Production deployment (PM2, Nginx, SSL, backups)
- Chunk 8: Frontend updates (subscription UI)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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