docs: add Databento integration plan and roadmap items

docs/roadmap/ROADMAP.yaml

@@ -1,5 +1,5 @@
 version: 1
-updated_at: 2026-03-27
+updated_at: 2026-03-28
 structure:
   backlog_dir: docs/roadmap/backlog
   in_progress_dir: docs/roadmap/in-progress
@@ -13,14 +13,20 @@ notes:
   - Pre-alpha policy: we may cut or replace old features without backward compatibility until alpha is declared.
   - Alpha migration policy: once alpha is declared, compatibility only needs to move forward; backward migrations are not required.
 priority_queue:
+  - DATA-DB-001
+  - DATA-DB-002
+  - DATA-DB-004
   - CONV-001
   - EXEC-002
+  - DATA-DB-003
+  - DATA-DB-005
   - DATA-002A
   - DATA-001A
   - OPS-001
   - BT-003
   - BT-002A
   - GCF-001
+  - DATA-DB-006
 recently_completed:
   - PORTFOLIO-003
   - PORTFOLIO-002
@@ -44,6 +50,12 @@ recently_completed:
   - CORE-002B
 states:
   backlog:
+    - DATA-DB-001
+    - DATA-DB-002
+    - DATA-DB-003
+    - DATA-DB-004
+    - DATA-DB-005
+    - DATA-DB-006
     - CONV-001
     - EXEC-002
     - DATA-002A