fix: correct output volume mount path to match OUTPUT_DIR default

Volume was mounted at /app/output but OUTPUT_DIR defaults to
./frigate_train (i.e. /app/frigate_train). Mount was never being used.

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

compose.yml

@@ -33,7 +33,7 @@ services:
       # Replace <pool> with your TrueNAS pool name, e.g. /mnt/tank/if-curator/...
       - /mnt/<pool>/if-curator/models:/models
       - /mnt/<pool>/if-curator/embeddings:/app/.if_cache
-      - /mnt/<pool>/if-curator/output:/app/output
+      - /mnt/<pool>/if-curator/output:/app/frigate_train
     stdin_open: true
     tty: true
     restart: unless-stopped