feat: add Invoice entity management feature

[devin-ai-integration]

Summary

Adds a full Invoice management feature following JHipster entity generation conventions. Users can create, view, edit, and delete invoices.

Backend (Spring Boot)

• Invoice JPA entity with fields: number (String), date (LocalDate), dueDate (LocalDate), amount (BigDecimal), status (enum: DRAFT/SENT/PAID/OVERDUE)
• ManyToOne relationship to BankAccount
• InvoiceRepository with eager-loading queries for the bankAccount relationship
• InvoiceService with full CRUD + partial update support
• InvoiceResource REST controller at /api/invoices with POST, PUT, PATCH, GET, DELETE endpoints
• Liquibase changelog for the invoice table with FK constraint to bank_account

Frontend (Angular)

• List view with sortable columns (number, date, due date, amount, status, bank account)
• Detail view showing all invoice fields with linked bank account
• Create/Edit form with validation for all required fields and bank account dropdown
• Delete confirmation dialog
• Entity routes, navbar menu entry, and i18n translations (English)

Tests

• InvoiceResourceIT: 20 integration tests covering CRUD, validation, partial updates, and error cases
• InvoiceTest: Domain entity equals verifier
• All 21 tests pass locally; ESLint and Checkstyle pass with 0 violations

Review & Testing Checklist for Human

• Verify the Invoice entity fields (number, date, dueDate, amount, status) meet business requirements
• Test creating an invoice via the Angular form — confirm all fields save correctly including the bank account relationship
• Test editing an existing invoice — confirm changes persist and status dropdown works
• Test deleting an invoice — confirm it's removed from the list
• Verify the Liquibase migration runs cleanly on a fresh database (H2 dev and PostgreSQL prod)

Notes

• The InvoiceStatus enum values are: DRAFT, SENT, PAID, OVERDUE
• Fake data CSV loads 3 sample invoices in dev profile
• The service layer is used (not @Transactional on the controller) to follow the pattern for entities with relationships

Link to Devin session: https://partner-workshops.devinenterprise.com/sessions/da219410568140ed8035b0faacefae83

[devin-ai-integration]

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