feat: run backfill_file_metadata as background job on pod startup

Launches the management command in the background immediately after
migrations so FileMetadata rows are populated for pre-existing files
without delaying pod readiness.  A comment marks the line for removal
once the fleet is fully backfilled.

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

docker/startup_scripts/start.sh

@@ -275,6 +275,11 @@ main() {
   setup_cache_dir
   fix_logging_and_socket_perms
 
+  # Backfill FileMetadata rows for files uploaded before MetadataFileField was
+  # deployed.  Runs as a background job so pod startup is not delayed.
+  # Once all instances have been fully backfilled this line can be removed.
+  python3 manage.py backfill_file_metadata --batch-size=200 --rate-limit=20 &
+
   cat <<'BANNER'
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