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/compound_fk.rs

@@ -276,7 +276,7 @@ fn compound_fk_declares_enforces_and_round_trips() {
         );
 
         // describe shows the compound endpoints symmetrically.
-        let city = db.describe_table("City".to_string(), None).await.unwrap();
+        let city = db.describe_table("City".to_string()).await.unwrap();
         let outbound = &city.outbound_relationships[0];
         assert_eq!(
             outbound.local_columns,
@@ -329,7 +329,7 @@ fn compound_fk_create_fk_makes_both_child_columns() {
         )
         .await
         .expect("add compound relationship with --create-fk");
-        let city = db.describe_table("City".to_string(), None).await.unwrap();
+        let city = db.describe_table("City".to_string()).await.unwrap();
         for col in ["c_country", "c_code"] {
             assert!(
                 city.columns.iter().any(|c| c.name == col),
@@ -527,7 +527,7 @@ fn compound_fk_survives_rebuild_from_text() {
             .await;
         assert!(bad.is_err(), "compound FK still enforced after rebuild from text");
         // Endpoints survived the round-trip intact.
-        let city = db.describe_table("City".to_string(), None).await.unwrap();
+        let city = db.describe_table("City".to_string()).await.unwrap();
         assert_eq!(
             city.outbound_relationships[0].other_columns,
             vec!["country".to_string(), "code".to_string()],
@@ -563,7 +563,7 @@ fn compound_fk_undo_removes_the_relationship() {
         .await
         .expect("add compound relationship");
         assert_eq!(
-            db.describe_table("City".to_string(), None)
+            db.describe_table("City".to_string())
                 .await
                 .unwrap()
                 .outbound_relationships
@@ -573,7 +573,7 @@ fn compound_fk_undo_removes_the_relationship() {
         // One undo step removes the whole relationship (ADR-0013/0006).
         db.undo().await.unwrap().expect("undo applied");
         assert!(
-            db.describe_table("City".to_string(), None)
+            db.describe_table("City".to_string())
                 .await
                 .unwrap()
                 .outbound_relationships