fix(db): prevent SQLite "database is locked" errors under concurrent writes

Configure SQLite connections with WAL journal mode, a 5-second busy
timeout, shared cache, and a max of 1 open connection. SQLite only
supports a single writer at a time, so without these settings concurrent
API requests (e.g. bulk task creation) would immediately fail with
"database is locked" instead of waiting and retrying.

.gitignore

@@ -9,6 +9,8 @@ config.yml.sample
 !.gitea/ISSUE_TEMPLATE/config.yml
 docs/themes/
 *.db
+*.db-shm
+*.db-wal
 Run
 dist/
 cover.*