Add a feature-flagged PostgreSQL solve-history read API

[coder13]

Goal

Expose durable per-user solve history from PostgreSQL only after the Room/RaceSession aggregate is authoritative. Do not build this API around the legacy room-embedded attempt model.

Read contract

• Require the authenticated user.
• Return cursor-paginated completed solves with stable solve ID, event, time, +2/DNF penalties, scramble, Room/RaceSession reference, and timestamps.
• Authorize private, hidden, expired, and deleted-room history server-side. Do not return passwords, access codes, room membership, or moderation data.
• Query the canonical PostgreSQL Solve → Attempt → RaceSession → Room relationship; Solve.room_id is only a temporary migration compatibility field and must not define API semantics.
• Use a session-aware deterministic index and cursor compatible with the final schema.
• Gate the client endpoint behind configuration and retain an explicit disable path, not a MongoDB fallback after aggregate cutover.
• Instrument latency and error/disable behavior without logging solve data or identity.

Acceptance criteria

• The API is authenticated, cursor-paginated, and stable across concurrent new solves.
• It returns the final Room/RaceSession-shaped representation and correct penalties/scrambles.
• Private-room, hidden/deleted-room, empty-history, disabled, and pagination authorization tests pass.
• Disabled PostgreSQL behavior is explicit and does not read MongoDB room/result state.
• Client history and csTimer-export consumers are documented against the final contract.

Dependencies

• Track competition rooms foundation and PostgreSQL cutover #199, including Define the Room and RaceSession domain contract #200, Add the PostgreSQL Room and RaceSession schema #203, Centralize room, RaceSession, attempt, and solve persistence #204, Backfill MongoDB data into PostgreSQL and verify ongoing parity #175, and Cut over rooms, RaceSessions, attempts, and solves to PostgreSQL #202.
• Stable production rollout from Rehearse and execute the modernized production rollout with a tested rollback #176 and settled room ownership/admin semantics from PR Stabilize room host ownership across rejoins #195.
• The PostgreSQL aggregate cutover and reconciliation must complete before implementation begins.

Unlocks

• Persist room solve history and past sessions #160
• Show previous scrambles for completed solves #161
• Export room sessions to csTimer #163