feat: explicit schema for all_events bronze table

[nkarpov]

Summary

• Define an explicit StructType schema on the all_events Auto Loader read so ts is ingested as TIMESTAMP and sequence as INT instead of inferred strings
• Remove the redundant to_timestamp("ts") cast in silver_order_items since ts is now natively typed
• Improves downstream query performance, especially for time-based filtering when synced to Postgres via reverse ETL

Test plan

• Run DLT pipeline and verify all_events schema shows ts as TIMESTAMP and sequence as INT
• Verify silver_order_items and gold tables populate correctly
• Confirm refund recommender stream handles the typed ts column without errors

This pull request was AI-assisted by Isaac.