GetSourceGroup errors in drainOne now gate on errors.Is(err,
store.ErrNotFound) before calling abandonPending, mirroring the
existing GetSchedule pattern. Transient errors (SQLITE_BUSY, context
cancellation) now log a warning and return without deleting the row.
Add regression test TestDrainPendingDropsRowsForGoneSourceGroup
confirming the ErrNotFound path still abandons correctly. Also add
a comment above the backoff-doubling loop explaining the progression.
steve
91fb7cf69b