Refactor: centralize Zod schemas and fully integrate into API routes

Improvements #1-3 deepening:

1. Server-side invoice amount validation
   - All amounts (qty × unitPrice) recalculated server-side using tax.ts
   - Prevents client-side manipulation attacks
   - Supports kleinunternehmer auto-inheritance

2. Comprehensive audit logging
   - LogAction type extended with 11 new actions
   - All CRUD operations now logged with metadata
   - Metadata includes: amounts, counts, status transitions, oldStatus/newStatus

3. Advanced Zod validation (centralized)
   - New file: app/lib/schemas.ts (220 lines, 18+ validators)
   - Custom validators: currencySchema, taxRateSchema, ibanSchema, taxIdSchema, vatIdSchema
   - All API routes (invoices, companies, customers) now use centralized schemas
   - Consistent German error messages
   - Single source of truth for validation logic

Additional improvements:
- DB indices applied: invoices(status, dueDate, deletedAt, customerId), customers(companyId)
- Migration 20260415192953_add_indices applied successfully
- Build succeeds without critical errors
- TypeScript compilation validates all schemas

Files modified:
- app/lib/schemas.ts (NEW)
- app/routes/api.invoices.ts (uses centralized schemas)
- app/routes/api.invoices.$id.ts (status transition validation)
- app/routes/api.companies.ts, api.companies.$id.ts
- app/routes/api.customers.ts, api.customers.$id.ts
- app/lib/logger.server.ts (metadata support)
- prisma/schema.prisma (indices)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

prisma/schema.prisma

@@ -159,6 +159,7 @@ model Customer {
   createdAt DateTime  @default(now())
   updatedAt DateTime  @updatedAt
 
+  @@index([companyId])
   @@map("customers")
 }
 
@@ -186,6 +187,10 @@ model Invoice {
   deletedAt         DateTime?
 
   @@unique([companyId, number])
+  @@index([status])
+  @@index([dueDate])
+  @@index([customerId])
+  @@index([deletedAt])
   @@map("invoices")
 }