feat(pending): app_name field and date-sharded pending storage

- Add optional app_name to PendingSubmission and prepare_submit_job,
  passed through PrepareSubmitJobRequest for user-provided tracking.
- Rewrite PendingStore to shard records by UTC date under
  data/pending_jobs/YYYY-MM-DD.json instead of a single monolithic
  pending_jobs.json.
- Legacy single-file pending_jobs.json remains readable; first save()
  migrates it and renames the old file to avoid double reads.
- Tests cover date sharding, multi-date list, legacy read, legacy
  migration, and app_name round-trip.

tests/unit/test_models.py

@@ -105,6 +105,7 @@ def test_pending_submission_defaults_to_pending_status():
     assert p.job_id is None
     assert p.application_id is None
     assert p.yarn_rm_url is None  # default for connections without one set
+    assert p.app_name is None
 
 
 def test_pending_submission_carries_yarn_rm_url_snapshot():