feat(practice): Practice Mode session — no competitive scoring, repeatable skill drills

Description

Practice Mode (formerly Tutorial Mode) is a permanent, repeatable content surface — not a one-time onboarding tunnel. Players can replay any map to improve their medal. This epic establishes the Practice Mode session model on top of the GameMode infrastructure.

Acceptance Criteria

• `GameMode.PRACTICE` sessions suppress position-based bonuses and competitive ranking
• The end screen shows a per-map medal result (Bronze / Silver / Gold / Diamond) instead of a podium
• Medal tiers use the same time-bracket thresholds as Race Mode (configurable, same defaults)
• Practice results are excluded from competitive leaderboards (filtered by `gameMode` in queries)
• Practice Mode supports single-player (minimum player count = 1, no countdown gate)
• Players can replay the same map/cup unlimited times; each attempt is recorded separately
• `GameResultPersistenceService` stores Practice results with `gameMode = PRACTICE`
• Unit tests cover medal assignment at each bracket boundary
• Unit tests verify Practice results are excluded from leaderboard queries

Technical Details

• Medal enum: `BRONZE`, `SILVER`, `GOLD`, `DIAMOND` — follows Enum DSL pattern
• End screen is a separate `PracticeEndPhase` (not shared with Race end screen)
• No DB schema changes beyond what Epic #167 introduces (`gameMode` column suffices)
• Leaderboard queries add `WHERE game_mode = 'RACE'` filter — no separate table

Dependencies

• #167 (GameMode enum and mode-aware session/orchestrator)

Estimate

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Start with the GameMode infrastructure and the mode-aware session/orchestrator from dependency #167, then trace GameResultPersistenceService and the leaderboard queries. Review the separate PracticeEndPhase design and existing Race Mode medal thresholds; done means Practice sessions support repeatable single-player attempts, persist PRACTICE results, show medals, and are excluded from competitive rankings with boundary and query tests.