H6 — Backend metadata DB (SQLite) & migrations

[sifterstudios]

Context
Persist sessions and song metadata (no audio bytes).

Acceptance Criteria

• Tables: Songs(hash PK, title, artist, durationMs, bpm), Sessions(code PK, createdAt), SessionSongs(session, index, hash), Answers(session, audienceId, songIndex, choiceId, correct).
• Migrations applied automatically on startup.
• Indexes on Answers(session,songIndex).

Tests

• Migration test on empty/new DB; CRUD smoke with in-memory SQLite.