refactor(db): unwind vestigial worker source plumbing (ADR-0052 follow-up)

ADR-0052 moved success journaling out of the worker to the dispatch
layer, leaving the `source` that handlers threaded purely for the
worker's old history.log write dead. Remove it:

- drop `_source` from finalize_persistence and do_rebuild_from_text
- inline + delete the three read-only *_request wrappers
- drop the now-unused `source` param from the ~30 forwarding worker
  handlers (leaf + composite), compiler-guided
- remove the `source` field from the DescribeTable/QueryData/RunSelect
  requests and their DatabaseHandle methods (call sites updated)

The only worker `source` left is the snapshot/undo label
(snapshot_then / stage_pre_mutation / begin_batch). Purely mechanical,
no behaviour change. 2471 pass / 0 fail / 1 ignored, clippy clean.

tests/it/iteration5_export_import.rs

@@ -362,7 +362,7 @@ fn end_to_end_export_then_import_real_project() {
 
     // Round-trip: the inserted row is back.
     let data_view = rt()
-        .block_on(async { imported_db.query_data("Customers".to_string(), None, None, None).await })
+        .block_on(async { imported_db.query_data("Customers".to_string(), None, None).await })
         .expect("query data");
     assert_eq!(data_view.rows.len(), 1);
     // Serial id auto-filled to 1; Name was the inserted value.