Backfill player identity and platform account links into Sprocket

[gankoji]

Summary

Backfill legacy player identity, Discord linkage, tracker links, and platform accounts into Sprocket-owned models.

Why

Even after write parity exists, historical player/account state must be present in Sprocket before legacy reads can be removed.

Source data

• mledb.player
• mledb.player_account
• mledb_bridge.player_to_player
• any existing Sprocket user/member/player linkage tables

Scope

• Build an idempotent backfill for historical player/account records.
• Preserve needed crosswalk information for cutover and auditability.
• Define conflict handling for duplicate platform IDs, missing Discord links, or ambiguous mappings.

Acceptance criteria

• Historical player/account state is materialized in Sprocket.
• The backfill is safe to rerun.
• Conflict cases are captured and reported.
• After backfill, account and player lookups no longer require legacy tables for migrated users.