chore: Database simplificactions

[SantiagoPittella]

closes #2351

1. Reorganized files: split the big utils.rs into migrations/settings/macros, moved store.sql into a migrations/ folder, renamed helpers.rs to rows.rs.
2. Read columns by name instead of number: row.get("nonce") instead of row.get(1), so reordering a query can't silently mix up values.
3. Removed copy-paste: small shared helpers now handle the repeated patterns: building IN-list parameters and the open-transaction/commit/rollback boilerplate.
4. Errors instead of crashes: a corrupted database now returns a proper error where it used to panic.
5. Deleted dead code and hid internal functions that were accidentally public.