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/tools/requests.py

@@ -33,6 +33,10 @@ class SaveConnectionRequest(BaseModel):
 
 class PrepareSubmitJobRequest(BaseModel):
     connection: str
+    app_name: str | None = Field(
+        default=None,
+        description="Optional human-readable application name for tracking the pending submission.",
+    )
     script_path: str = Field(
         ...,
         description=(