Release savepoints after rollback

[ShiroKSH]

Summary

• release a savepoint after rolling it back
• keep rollback and release as separate SQLite statements
• cover nested cleanup and successful transaction reuse after a rollback

Root cause

ROLLBACK TO SAVEPOINT rewinds database changes but leaves the savepoint active. The connection therefore remained inside a transaction after a throwing outermost savepoint block, causing the next transaction to fail with cannot start a transaction within a transaction.

Impact

Failed savepoint blocks now leave the connection ready for subsequent transactions. Normal transaction and successful savepoint behavior are unchanged.

Validation

• git diff --check
• targeted SQLite state-transition reproduction for nested rollback/release cleanup and subsequent transaction reuse

[ShiroKSH]

@stephencelis We'd appreciate it if you could take a look at this pull request when you have a chance.