Add database constraint and application validation for `play_status`

[jackmanuel]

Description

The play_history.play_status column currently accepts any text value, although the bot only intentionally uses:

• queued
• playing
• completed
• skipped

Add validation to prevent typos or future code paths from storing unsupported statuses.

Proposed changes

• Add a SQLite CHECK constraint:

CHECK (play_status IN ('queued', 'playing', 'completed', 'skipped'))

• Validate status values in DatabaseManager.update_play_status() and raise a clear error for invalid values.
• Audit existing databases for unexpected values before migration.
• Rebuild the table to apply the constraint, since SQLite cannot add it directly to an existing column.
• Preserve all existing data, indexes, defaults, and column types during migration.
• Add tests confirming that valid statuses are accepted and invalid statuses are rejected.

Acceptance criteria

• Existing databases migrate without losing valid data.
• All four supported statuses continue to work.
• Invalid status values cannot be inserted or assigned.
• Existing indexes are recreated after migration.
• The migration handles unexpected legacy values safely and reports them clearly.