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/show_list.rs

@@ -462,7 +462,7 @@ fn app_show_table_renders_relationships_as_compact_diagrams() {
     rt.block_on(seed_schema(&db));
     // Orders holds the FK to Customers — an outbound relationship.
     let desc = rt
-        .block_on(db.describe_table("Orders".to_string(), None))
+        .block_on(db.describe_table("Orders".to_string()))
         .expect("describe Orders");
 
     let mut app = App::new();