[IP-160]: convert quote to invoice

[nielsdrost7]

Summary

• QuoteService::convertQuoteToInvoice(Quote): Invoice — inside a transaction: copies prospect_id → customer_id, all quote items (name, qty, price, discounts, both tax rates, task link), the discount/tax/total summary, and summary/terms/footer onto a new draft invoice; then sets the quote to the new QuoteStatus::CONVERTED case.
• The draft is created with a null invoice number — consistent with the duplicate-prevention observer (PR [IP-513]: duplicate-number prevention via Observers #540) that allows multiple nulls for drafts; numbering is assigned when the invoice leaves draft.
• Converting twice throws and is surfaced as a danger notification; the convert_to_invoice table action (existing stub, now wired) hides for already-converted quotes and requires confirmation.

Deviation from the SMART plan: no redirect to EditInvoice after conversion — that page didn't exist on develop when this PR was written. It does now (further extended by PR #575), so the redirect is a ready-to-do follow-up (#588).

Closes

Closes #160 — quote-to-invoice conversion implemented end-to-end (service, action, status transition)

Follow-up issues (not resolved here — reopen if auto-closed on merge)

Closes #588 — Redirect to the invoice edit page after converting a quote (now unblocked since EditInvoice exists on develop)

Tests

QuoteConversionTest — 3 tests: full conversion (items, totals, status), double-conversion rejection, action visibility.