Backfill legacy schedule approvals and scheduled times into Sprocket

[gankoji]

Summary

Backfill historical scheduling approvals and approved scheduled times from legacy scheduling records into Sprocket.

Why

Once Sprocket owns scheduling writes, historical scheduled-time data still needs to be migrated so that captain scheduling history and downstream consumers can move off legacy joins.

Source data

• mledb.series.scheduled_time
• mledb_bridge.series_to_match_parent
• any schedule approval metadata currently stored in bot-side Redis or recoverable audit trails

Scope

• Backfill scheduled times onto the Sprocket-side scheduling model.
• Define how to handle missing bridge rows or ambiguous mappings.
• Migrate whatever approval-state history is practical to preserve.

Acceptance criteria

• Historical scheduled times are present in Sprocket for bridged matches.
• The backfill is safe to rerun.
• Missing/ambiguous mappings are reported.
• Sprocket can serve as the historical source for approved match times.