Invoice Generator

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Build me an invoicing tool that replaces the online invoice generators: a proper invoice, numbered correctly, that I still have in five years.

STACK
- Node 20+ with Fastify, server-rendered HTML
- SQLite through better-sqlite3, WAL mode
- PDF generated server-side from HTML with a headless browser, or with a PDF library if you want no browser on the box
- Caddy in front

THE DATA MODEL
- businesses: id, name, address, tax_id, vat_number, iban, logo_path, default_currency, default_terms, invoice_prefix, next_number
- clients: id, business_id, name, address, tax_id, vat_number, email, currency, payment_terms_days, notes
- invoices: id, business_id, client_id, number, issued_on, due_on, currency, status, subtotal_cents, tax_cents, total_cents, notes, terms, pdf_path, pdf_hash, sent_at, paid_at, voided_at, void_reason
- lines: id, invoice_id, position, description, quantity, unit_price_cents, tax_rate, discount_percent
- payments: id, invoice_id, amount_cents, paid_on, method, reference
- number_sequences: business_id, year, last_number — numbering is a table, not a max() query
- Every amount in integer cents with its currency. Never a float. Say why in a comment: 0.1 + 0.2 is the oldest bug in billing

NUMBERING, WHICH IS A LEGAL MATTER IN MOST COUNTRIES
- Sequential with no gaps, per business and per year, allocated inside the transaction that creates the invoice
- A draft has no number; a number is taken only when the invoice is issued
- An issued invoice is never deleted and never edited. A mistake is voided with a reason and replaced by a credit note that references it
- A credit note is an invoice with negative lines and its own sequence
- The rules differ by country: the README says which assumptions were made, and the sequence format is configurable

TAX
- Per-line tax rate, and the totals broken out by rate — most tax authorities require the breakdown, not just the total
- Reverse charge for EU cross-border business customers, with the required sentence printed and the VAT number of both parties shown
- Zero-rate and exempt are different things and are stored differently
- A tax rate is stored on the line at the time of issue. Changing a rate later must never alter an invoice already issued

THE DOCUMENT
- One template that looks like an invoice a bookkeeper has seen before: issuer, client, number, dates, lines, tax breakdown, total, payment details, terms
- The PDF is generated once at issue and stored, with its hash. Regenerating must produce the same bytes or the original is served — an invoice that changes after being sent is not an invoice
- A public link to view it, unguessable and revocable
- Multi-currency, with the exchange rate and its date recorded if the accounts are kept in another currency

GETTING PAID
- Payment details on the document: bank transfer, or a link to a payment page
- Partial payments recorded, with the balance shown
- Status derived, never typed: draft, issued, overdue, partly paid, paid, void
- Reminders before and after the due date, on a schedule the business sets, each one recorded so nobody is chased twice in a day

RECURRING
- A template and a cadence; the worker issues the next one and emails it
- Idempotent: the worker can run twice without issuing twice
- A recurring series can be paused, and pausing does not renumber anything

WHAT COMES OUT
- Export every invoice as PDF and the data as CSV or JSON, per period
- A report by client, by month, by tax rate
- Everything must be exportable in one command, because the point of leaving a SaaS is not being locked into the next one

THE INTERFACE
- One screen to write an invoice: client, lines, done. Totals update as you type
- The list shows number, client, total, status, due date
- Dark and light, one accent, and a print stylesheet that matches the PDF

OPERATIONS
- .env: DATABASE_PATH, PDF_PATH, BASE_URL, SMTP_URL, SESSION_SECRET
- Migrations on boot, each once
- Backup covers the database and the PDF directory, and the restore is tested
- Health endpoint that checks disk space and renders a one-line PDF

WHAT MATTERS MOST
Numbering and immutability. Build the sequence allocation, the issue transaction and the stored PDF first, and try to break them: two invoices issued in the same millisecond, a voided one, a year boundary. Everything else is a form — but an invoice with a duplicate number is a conversation with an accountant.

Give me the repository, migrations, .env.example, a seed business with three clients and five invoices, and a README with deploy steps behind Caddy and a note on what to check with a local accountant.