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- When body.tipSplits is provided in PATCH /invoices/:id, validate sum first then atomically replace existing splits (delete + insert) - When no incoming splits, validate existing DB splits with corrected message: "Tip splits are required when tip amount is greater than zero" (previously misleading "must sum to 100%" when no splits existed) Co-Authored-By: Paperclip <noreply@paperclip.ing>