Add a feature-flagged PostgreSQL solve-history read API

[coder13]

Goal

Expose durable per-user solve history from PostgreSQL as the first controlled read path, enabling #160, #161, and #163 without switching live room reads away from MongoDB.

Acceptance criteria

• Define an authenticated, cursor-paginated API for the current user's completed solves.
• Return stable solve IDs, event, time, +2/DNF penalties, scramble, room/session reference, and timestamps while enforcing private-room expectations.
• Use the existing creation-time + solve-ID index for deterministic pagination.
• Gate the read path behind configuration and provide an explicit fallback/disable path.
• Add authorization, pagination, penalty, empty-history, and disabled-PostgreSQL tests.
• Instrument latency, error rate, and fallback/disable behavior without logging solve data or identity.
• Document how the client history UI and csTimer export should consume the API.

Dependencies

• Persist room solve history and past sessions #160
• Backfill MongoDB data into PostgreSQL and verify ongoing parity #175
• 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