fix(ai): cap extract-invoices batch at 20 files

Final review flagged that the multipart limits set only fileSize, so a
single /extract-invoices request could carry unbounded PDF parts. The
per-batch budget re-check already bounds total spend, but this bounds the
work (one vision call per file) of any single request.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/routes/admin/ai.ts

@@ -19,7 +19,9 @@ import {
 
 export default async function aiRoutes(app: FastifyInstance): Promise<void> {
   await app.register(multipart, {
-    limits: { fileSize: config.nas.maxUploadSize },
+    // Cap files per request: the budget re-check already bounds *spend* mid-batch,
+    // but this bounds the work (one vision call per file) of a single request.
+    limits: { fileSize: config.nas.maxUploadSize, files: 20 },
   });
 
   // GET /api/admin/ai/usage — current-month spend + budget