feat(saas): update schema for db-per-tenant and per-component FIEL encryption

- Rename Tenant.schemaName to databaseName across all services
- Add Subscription and Payment models to Prisma schema
- Update FielCredential to per-component IV/tag encryption columns
- Switch FIEL encryption key from JWT_SECRET to FIEL_ENCRYPTION_KEY
- Add Subscription and Payment shared types
- Update JWTPayload to use databaseName

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/lib/api/tenants.ts

@@ -5,7 +5,7 @@ export interface Tenant {
   nombre: string;
   rfc: string;
   plan: string;
-  schemaName: string;
+  databaseName: string;
   createdAt: string;
   _count?: {
     users: number;