fix(sql): add ORDER BY to all LIMIT queries for deterministic results

SQLite does not guarantee row order without ORDER BY, even with LIMIT.
This was a systemic issue found during a multi-pass bug hunt:

Production queries (explain.rs):
- Outgoing reference query: ORDER BY target_entity_type, target_entity_iid
- Incoming reference query: ORDER BY source_entity_type, COALESCE(iid)
  Without these, robot mode output was non-deterministic across calls,
  breaking clients expecting stable ordering.

Test helper queries (5 locations across 3 files):
- discussions_tests.rs: get_discussion_id()
- mr_discussions.rs: get_mr_discussion_id()
- queue.rs: setup_db_with_job(), release_all_locked_jobs_clears_locks()
  Currently safe (single-row inserts) but would break silently if tests
  expanded to multi-row fixtures.

src/ingestion/mr_discussions.rs

@@ -786,8 +786,12 @@ mod tests {
     }
 
     fn get_mr_discussion_id(conn: &Connection) -> i64 {
-        conn.query_row("SELECT id FROM discussions LIMIT 1", [], |row| row.get(0))
-            .unwrap()
+        conn.query_row(
+            "SELECT id FROM discussions ORDER BY id LIMIT 1",
+            [],
+            |row| row.get(0),
+        )
+        .unwrap()
     }
 
     #[allow(clippy::too_many_arguments)]