fix: fourge_error revisions always bill $0 to client

Fourge's own error revisions should never be charged to the client.
getRevisionChargeQuantity now returns 0 for revision_type=fourge_error
regardless of version number. Applied at all 4 invoice picker call sites.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/invoiceVersionRules.js

@@ -12,7 +12,9 @@ export function isInitialVersionEligible(task) {
   return isCompletedVersionEligible(task, 0);
 }
 
-export function getRevisionChargeQuantity(versionNumber) {
+export function getRevisionChargeQuantity(versionNumber, revisionType) {
+  // Fourge's own error revisions are never billed to the client.
+  if (revisionType === 'fourge_error') return 0;
   const version = Number(versionNumber || 0);
   return version >= 2 ? 1 : 0;
 }