fix(db): distinguish database open failures

[Tomauskasz]

Summary

• Report native addon load failures as SQLITE_RUNTIME_ERROR and retain the original cause.
• Report locked, unreadable, and other availability failures as DB_OPEN_ERROR.
• Reserve INVALID_DB for SQLite format and corruption errors.
• Document the revised error contract.

The previous catch block converted every better-sqlite3 open failure into INVALID_DB. This hid native ABI failures such as a Node runtime requiring NODE_MODULE_VERSION 147 while the installed addon was built for 137, and incorrectly blamed the selected database.

Verification

• npm run build
• npm test — 12 test files, 50 tests passed
• npm exec -- tsc --noEmit -p tsconfig.json
• git diff --check