Add the PostgreSQL Room and RaceSession schema

[coder13]

Part of #199.

Goal

Reshape the canonical Prisma/PostgreSQL model so rooms can own multiple independent race sessions without changing production read ownership yet.

Schema direction

• Keep room identity, access, lifecycle, ownership, and optional competition association on app.rooms.
• Add app.race_sessions with room, event, format/mode, scramble source, state, scheduled times, and actual start/end times.
• Move current racing fields such as cube_event, started, and next_solve_at from their conceptual ownership on Room to RaceSession.
• Add session-level participant state for competing, waiting, and eligibility/registration where required by the domain contract.
• Make attempts reference race_session_id.
• Keep solves related through attempts; remove or deprecate redundant room_id only through an expand-and-contract migration.
• Preserve existing stable solve, attempt, room, and user identifiers.
• Add the minimal competition association defined by the domain contract.

Migration requirements

• Use additive Prisma migrations first.
• Do not perform a destructive migration in application startup.
• Keep the currently deployed application compatible during the expansion phase.
• Add appropriate foreign keys, uniqueness constraints, and indexes for:
  • active sessions by room;
  • session attempt ordering;
  • user solve history;
  • competition-room session lookup;
  • migration reconciliation.
• Define explicit delete/archive behavior. Historical solves must not cascade away because a room is hidden or archived.

Acceptance criteria

• Prisma models and committed migrations represent the approved domain contract.
• Existing PostgreSQL data remains readable during rollout.
• A fresh PostgreSQL 17 database applies every migration with zero drift.
• Upgrade tests run from the current production schema.
• Foreign-key and uniqueness constraints reject invalid cross-room/session relationships.
• Indexes cover active room/session and history read paths.
• No migration deletes historical attempts or solves.
• Rollback compatibility and the later contract phase are documented.

Dependencies

• The contract in Track competition rooms foundation and PostgreSQL cutover #199 and its dedicated domain-contract issue must be settled first.