A run referencing an undefined stack (or other env-config error) caused
resolve_for_task to raise ValueError. In submit_task this call sat outside
the try block, so the error propagated through process_run ->
restore_from_storage -> the FastAPI lifespan, aborting startup entirely —
one bad run on disk took down the whole server.

- submit_task: wrap env resolution + script generation; mark the task
  FAILED on ValueError instead of throwing.
- restore_from_storage: wrap process_run as defense-in-depth so no single
  run can abort startup.

Bump version to 0.11.3.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>