feat(audit): truncate stdout/stderr to 64KB + index FlowRun(user_id, started_at desc)

- Truncate accumulated stdout/stderr to last 64KB before persisting FlowStep
  to prevent unbounded DB growth on verbose commands
- Add @@index([user_id, started_at(sort: Desc)]) to FlowRun schema so audit
  list queries (WHERE user_id = ? ORDER BY started_at DESC) use the index
- Add migration 20260513200000_flowrun_user_idx

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

prisma/schema.prisma

@@ -42,6 +42,8 @@ model FlowRun {
   exit_code  Int?
   user       User       @relation(fields: [user_id], references: [id], onDelete: Cascade)
   steps      FlowStep[]
+
+  @@index([user_id, started_at(sort: Desc)])
 }
 
 model FlowStep {