fix(GRO-785): address invoice tip split regression

- Use body.tipCents ?? current.tipCents for validation condition
  so that simultaneous status=paid + tipCents=0 skip split validation
- Use body.tipCents (now aliased as tipCents) instead of current.tipCents
  inside the atomic transaction for shareCents calculation
- Add explicit check for empty tipSplits array with appropriate error
  message ("Tip splits are required when tip amount is greater than zero")
  before the sum-to-100% check
- Destructure tipSplits out of body before spreading into update object
  to prevent it from leaking into the invoices table SET clause

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/routes/invoices.ts

@@ -350,20 +350,25 @@ invoicesRouter.patch(
     }
 
     // Validate and persist tip splits when marking invoice as paid
-    if (body.status === "paid" && current.tipCents > 0) {
+    const tipCents = body.tipCents ?? current.tipCents;
+    if (body.status === "paid" && tipCents > 0) {
       // If incoming splits are provided in the request body, atomically replace them
       if (body.tipSplits !== undefined) {
+        if (body.tipSplits.length === 0) {
+          return c.json({ error: "Tip splits are required when tip amount is greater than zero" }, 400);
+        }
         const totalPct = body.tipSplits.reduce((sum, s) => sum + s.sharePct, 0);
         if (Math.abs(totalPct - 100) > 0.01) {
           return c.json({ error: "Tip split percentages must sum to 100%" }, 400);
         }
         await db.transaction(async (tx) => {
           await tx.delete(invoiceTipSplits).where(eq(invoiceTipSplits.invoiceId, id));
-          if (body.tipSplits.length > 0) {
-            let remaining = current.tipCents;
-            const rows = body.tipSplits.map((s, i) => {
-              const isLast = i === body.tipSplits.length - 1;
-              const shareCents = isLast ? remaining : Math.round((s.sharePct / 100) * current.tipCents);
+          const splits = body.tipSplits!;
+          if (splits.length > 0) {
+            let remaining = tipCents;
+            const rows = splits.map((s, i) => {
+              const isLast = i === splits.length - 1;
+              const shareCents = isLast ? remaining : Math.round((s.sharePct / 100) * tipCents);
               if (!isLast) remaining -= shareCents;
               return {
                 invoiceId: id,
@@ -400,7 +405,10 @@ invoicesRouter.patch(
       }
     }
 
-    const update: Record<string, unknown> = { ...body, updatedAt: new Date() };
+    // Destructure tipSplits out — it belongs to a separate table, not the invoices column
+    // eslint-disable-next-line @typescript-eslint/no-unused-vars
+    const { tipSplits: _tipSplits, ...updateBody } = body as Record<string, unknown>;
+    const update: Record<string, unknown> = { ...updateBody, updatedAt: new Date() };
 
     // Auto-set paidAt when marking as paid
     if (body.status === "paid" && !body.paidAt && !current.paidAt) {