Tech debt: migration check on startup

Tracked in `roadmap.md` *Tech debts* as: **"migration check on startup"**

The server starts happily against a database whose schema is behind the migrations registered in `templates/backend/internal/database/migrations/`. It should detect the drift at boot.

Needs a decision on the response: hard-fail, warn-and-continue, or auto-apply (and whether that differs between dev and production).

Start with roadmap.md and inspect the migration files under templates/backend/internal/database/migrations/. First resolve whether startup should hard-fail, warn and continue, or auto-apply migrations, including any development-versus-production difference. Done means the server detects when the database schema is behind the registered migrations according to the chosen behavior.

Written by the indexing model from the issue text.