MIND-11: Persist session log in SQLite

Summary

Log completed meditation sessions and intervals to a local SQLite database.

Tasks

• Set up SQLite integration (sqflite or drift).
• Define schema: sessions, intervals (based on DATA_MODEL.md).
• Implement migration strategy using PRAGMA user_version.
• Save completed sessions and intervals in a transaction.
• Add DAO methods for inserts and queries.

Acceptance Criteria

• Database is created and initialized on first run.
• Completed sessions persist correctly with duration and intervals.
• Data survives app restarts.