Drops the isInt({ min: 0 }) constraint on lineItems.*.unitPriceMinor
in both the adminInvoices and adminQuotes POST/PUT validators so
admins can add Treuerabatt / Frühbucherrabatt rows as standalone
negative-priced lines (matches standard DE/CH invoice practice).
A service-layer guard rejects saves whose computed total goes below
zero (INVOICE_TOTAL_NEGATIVE / QUOTE_TOTAL_NEGATIVE, both 400) so a
mis-typed discount can't accidentally mint a credit-balance invoice
that would masquerade as a regular row in dashboards. Credit notes
still belong in the Storno path (createStorno), which is unchanged.
Quote-side integration coverage is omitted for now — createQuote's
cold-require path takes ~30s under the test harness; the invoice
test exercises the same validator + guard shape.
