Unify database migration system

[DylanDevelops]

The feature request

The database initialization currently mixes two different schema management patterns. storage.Initialize() executes ad-hoc ALTER TABLE queries and relies on string-matching errors ("duplicate column name") to skip existing columns, while an actual migration system exists in migrations.go for timezone updates.

Proposed solution

Fold the ALTER TABLE modifications into the numbered migration system. This will ensure CurrentSchemaVersion accurately reflects the state of the database and removes fragile error-string parsing from the initialization sequence.

Additional context

For safety, an automatic VACUUM INTO backup should be triggered right before applying new migrations.