useSnapshotEditor.save now validates all simple/priced lines in-memory
before any DB write, then delegates to a new saveSnapshotAtomic helper
that wraps INSERT snapshot + INSERT lines in an explicit BEGIN/COMMIT
transaction (ROLLBACK on catch). Pattern matches categorizationService.
Migration v11 cleans existing orphan snapshots in profiles that hit the
old race; new orphans are no longer possible thanks to the transaction.
Resolves#176
