[feature]: Prevent Switching from Native SQL to KV SQL After Invoice Migration

[bhandras]

Description
Following the changes introduced in PR #8831, it would be desirable to prevent users from switching back from native SQL to KV SQL after the invoice migration process. This is crucial to avoid potential side effects such as lingering invoices or partial state being stored in KV/native tables.

Proposed Solution
Introduce a mechanism to detect if the invoice migration has already occurred. A possible approach is to add a "tombstone" key to the KV top-level invoices bucket. This key would act as a marker indicating that the migration has been completed. Upon startup, the system can check for this marker, and if it exists, prevent the user from manually overriding the migration.

Steps

• Add a "tombstone" key to the KV top-level invoices bucket during the invoice migration process.
• Implement a check during startup to verify if the "tombstone" exists.
• If the "tombstone" is present and the user attempts to switch back to KV SQL, block the operation and log an appropriate error message.