feat: idempotent handlers, deterministic activity ids

crates/adapters/image-converter/src/handler.rs

@@ -45,6 +45,17 @@ impl EventHandler for ImageConversionHandler {
             return Ok(());
         }
 
+        // `get` falls back to `{key}.webp` / `{key}.avif`, which would re-encode
+        // an already converted image on redelivery. Probe the exact key first:
+        // once the original is gone, an earlier delivery finished the conversion.
+        match self.storage.get_stream(&key).await {
+            Ok(_) => {}
+            Err(DomainError::NotFound(_)) => {
+                tracing::debug!("original {key} already converted, skipping");
+                return Ok(());
+            }
+            Err(e) => return Err(e),
+        }
         let bytes = self.storage.get(&key).await?;
         let format = self.format;