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.

spark_executor/models.py

@@ -86,6 +86,7 @@ class Connection(BaseModel):
 
 class PendingSubmission(BaseModel):
     pending_id: str
+    app_name: str | None = None
     connection: str
     master: str
     deploy_mode: str