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_create_index.rs

@@ -55,7 +55,7 @@ fn insert_row(db: &Database, r: &tokio::runtime::Runtime, id: i64, email: &str)
 }
 
 fn index(db: &Database, r: &tokio::runtime::Runtime, name: &str) -> Option<(Vec<String>, bool)> {
-    r.block_on(db.describe_table("T".to_string(), None))
+    r.block_on(db.describe_table("T".to_string()))
         .expect("describe")
         .indexes
         .into_iter()