fix: use iter() for Kafka since FutureRecord borrows data

FutureRecord borrows from the tuple values (topic, key, payload).
Using into_iter() would move these into the closure, but the async
block can't outlive these local variables. Using iter() keeps the
prepared Vec alive while futures execute.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: psteinroe

src/sink/kafka.rs

@@ -209,10 +209,12 @@ impl Sink for KafkaSink {
             .collect::<EtlResult<Vec<_>>>()?;
 
         // Send all messages concurrently.
+        // Note: FutureRecord borrows from the tuple values, so we must use iter()
+        // to keep prepared alive while the futures execute.
         let futures: Vec<_> = prepared
-            .into_iter()
+            .iter()
             .map(|(topic, key, payload)| {
-                let record = FutureRecord::to(&topic)
+                let record = FutureRecord::to(topic)
                     .key(key.as_str())
                     .payload(payload.as_slice());
                 let producer = &self.producer;