feat: Recurring invoices + license suspend on dunning level 3 (PRD §6.4/§6.6)

- Recurring API: list contracts needing invoicing + run endpoint
- Auto-generate draft invoices for Wartung/SLA + Dienstvertrag contracts
- Automatic license suspension at dunning level 3 (PRD §6.4)
- Recurring billable expenses tracking
- MRR/ARR computation from contracts + expenses
- Prisma: added Contract.company + Expense.company relations
- Scheduler info (pg-boss monthly cron in production)

prisma/schema.prisma

@@ -81,6 +81,8 @@ model Company {
   licenses      License[]
   invoices      Invoice[]
   projects      Project[]
+  contracts     Contract[]
+  expenses      Expense[]
 }
 
 model Person {
@@ -300,6 +302,7 @@ model Contract {
   id            String   @id @default(cuid())
   number        String   @unique // gapless, e.g. VER-2026-0001
   companyId     String
+  company       Company  @relation(fields: [companyId], references: [id])
   type          String   // werkvertrag|dienstvertrag|wartung-sla|avv|nda|dpa
   title         String
   status        String   @default("draft") // draft|sent|signed|active|expired|terminated
@@ -335,6 +338,7 @@ model Expense {
   date          DateTime @default(now())
   // linking
   companyId     String?  // customer this is billable to
+  company       Company? @relation(fields: [companyId], references: [id])
   projectId     String?  // project this relates to
   billable      Boolean  @default(false) // convert to customer invoice
   markupPct     Int      @default(0) // markup when re-billed