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/sql_drop_table.rs

@@ -150,7 +150,7 @@ fn drop_table_is_one_undo_step_and_restores_data() {
     assert!(r.block_on(db.undo()).expect("undo").is_some(), "the drop was one undo step");
     assert!(r.block_on(db.list_tables()).unwrap().contains(&"T".to_string()));
     let data = r
-        .block_on(db.query_data("T".to_string(), None, None, None))
+        .block_on(db.query_data("T".to_string(), None, None))
         .expect("query");
     assert_eq!(data.rows.len(), 1, "the dropped row was restored by undo");
 }