Migrate challenge progress storage to SQLite

[minoneer]

Summary

• move challenge progress from legacy YAML files into a SQLite-backed repository
• migrate legacy completion and player challenge data eagerly on first startup into island-owned records
• keep migrated backups under backup/completion and store the DB under data/challenge-progress.db

Notes

• legacy import is one-shot and tracked in DB metadata so startup does not keep rescanning old files forever
• the repository keeps sparse progress rows and uses WAL mode with a short busy timeout

Testing

• ./gradlew :uSkyBlock-Core:test --tests us.talabrek.ultimateskyblock.challenge.ChallengeCompletionLogicTest --tests us.talabrek.ultimateskyblock.challenge.SqliteChallengeProgressRepositoryTest --tests us.talabrek.ultimateskyblock.island.IslandKeyTest