SQL store hardening - Internal cleanup

* **Replace positional row reads?** Around 70 reads use `row.get(0)` beside hand-written column lists, so column reordering can map values incorrectly. I'm not sure there's too much value in this, as long as queries are wlel tested
* Use named columns or per-table row mappers with canonical column-list constants.
* **Centralize integer conversions.** Route remaining `as i64` casts through the shared `u64`/`i64` helpers.
* **Split migration and settings code.**
* Move `store.sql` into `migrations/`.
* Split `db_management/utils.rs` into migration, settings, and macro modules.
* Rename `account/helpers.rs` to `account/rows.rs` and place row mappers there.

Look at the SQL store files, particularly store.sql and db_management/utils.rs. Identify the 70+ positional row reads (row.get(0)) and replace them with named columns or row mappers. Centralize integer conversions using existing u64/i64 helpers. Split the migration and settings code by moving store.sql to migrations/ and refactoring utils.rs. Rename account/helpers.rs to account/rows.rs for row mappers. Run existing database tests to ensure correctness.

Scritto dal modello di indicizzazione a partire dal testo della issue.