The database was never properly closed on SIGINT/SIGTERM — WAL files were left
in a partially-written state causing "database disk image is malformed" errors.
Adds WAL checkpoint on close, synchronous=NORMAL pragma, SIGTERM handlers, and
a store.close() method that flushes all pending writes before exit.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>