Two categories of fixes:
1. Use defer s.Close() instead of explicit s.Close() to prevent session
leaks when require.FailNow() triggers runtime.Goexit(), which skips
explicit close calls but runs deferred functions. Leaked sessions
hold SQLite write locks that block all subsequent fixture loading.
2. Add s.Commit() before db.AssertExists/db.AssertMissing calls. These
assertion helpers query via the global engine (not the test session),
so they cannot see uncommitted data from the session's transaction.
For block-scoped sessions (kanban_task_bucket_test.go), wrap each block
in an anonymous function so defer runs at block boundary rather than
deferring to the enclosing test function.
