Add upgrade compatibility tests for database migrations #1637

Context

The migration PR (#1561) replaces GORM with golang-migrate + sqlc. Manual testing confirmed upgrade safety, but we need automated tests to prevent regressions as the schema evolves.

Tests to add

1. Rolling upgrade compatibility: previous release code against new schema

Verify that the previous release's code can read and write against the current version's schema. This simulates a rolling deploy where old pods are still running while the new schema has already been applied.

2. Data-preserving upgrade: previous release with data to current version

Verify that an upgrade from the previous release preserves core existing data.