SQL store hardening - Errors

* **Expose structured SQL failures.** Callers currently receive opaque strings and cannot distinguish retryable lock contention from corruption.
* Add variants for busy or locked databases.
* Add a constraint-violation variant.
* Add a corruption variant.
* **Make settings row-count failures consistent.** `set_setting` and `remove_setting` currently panic in debug builds and silently continue in release builds.
* Return an error when the affected row count is not one.

Look at the SQL store implementation, likely in a module like `store/sql.rs` or similar. Examine how errors are currently returned as strings and identify where SQL operations (like setting/removing settings) are performed. The goal is to define new error variants (e.g., `SqlError::Busy`, `SqlError::ConstraintViolation`) and replace panics with proper error returns. Check for existing tests around SQL operations to understand the expected behavior.

Written by the indexing model from the issue text.