Backfill MongoDB data into PostgreSQL and verify ongoing parity

[coder13]

Part of #199.

Goal

Turn PostgreSQL dual writes into a trustworthy migration target by copying historical MongoDB data and continuously checking parity.

For the competition-room foundation, this backfill must target the new Room/RaceSession model rather than preserving the current assumption that a room is itself one racing session.

RaceSession mapping

• Convert each existing MongoDB room into one PostgreSQL Room plus one RaceSession for its current event.
• Attach every embedded attempt to that RaceSession while preserving deterministic attempt and solve IDs.
• Populate session participant state according to the approved domain contract.
• Preserve room identity, access code, ownership, privacy, timestamps, and deletion/expiration state.
• Make the mapping explicit and repeatable; do not attempt to infer event sessions whose attempts were already deleted by the legacy event-change behavior.
• Verify concurrent live writes are neither lost nor duplicated during the backfill.

Acceptance criteria

• Implement an idempotent, resumable backfill for users/preferences, rooms, race sessions, participant state, attempts, durable solve results, and sanitized metrics.
• Support dry-run, bounded batches, progress checkpoints, and safe retries.
• Do not copy OAuth access tokens or prohibited metric fields.
• Produce reconciliation output for record counts, missing identifiers, duplicate identifiers, room/session relationships, penalty fields, and representative aggregates.
• Verify every attempt belongs to the expected RaceSession and every RaceSession belongs to the expected Room.
• Verify writes arriving during the backfill are not lost or duplicated.
• Document operational invocation, expected runtime/load, alerts, failure recovery, and how to rerun only failed ranges.
• Add focused tests for deterministic IDs, cursor boundaries, retry behavior, session mapping, and reconciliation failures.

Dependencies

• Stable production dual writes from PR Add PostgreSQL dual writes and room metrics #171, deployed through Rehearse and execute the modernized production rollout with a tested rollback #176.
• The Room/RaceSession domain contract and expansion schema from Define the Room and RaceSession domain contract #200 and Add the PostgreSQL Room and RaceSession schema #203.
• The schema and migration history must remain compatible with rollback requirements.

Unlocks

• Cut over rooms, RaceSessions, attempts, and solves to PostgreSQL #202
• Add a feature-flagged PostgreSQL solve-history read API #174
• Persist room solve history and past sessions #160