feat: implement Story 1.4 — recent view and drag-and-drop organization

Add sortOrder column to diagrams, extend PATCH endpoint with projectId
and sortOrder fields, add POST /diagrams/reorder bulk endpoint with
ownership verification and duplicate ID validation. Enhance RecentList
with lastAiMessage preview subtitle. Implement @dnd-kit drag-and-drop
in ProjectTree sidebar with cross-project moves, intra-project reorder,
optimistic updates, drag overlay, drop indicators, and keyboard/pointer
sensor support. Context-aware GET ordering: sortOrder for project views,
updatedAt for recent/all views. 141 tests pass.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/api/tests/diagram/diagram-db-schema.test.ts

@@ -65,6 +65,7 @@ describe("selectDiagramSchema", () => {
       graphData: {},
       userId: "user-123",
       projectId: null,
+      sortOrder: 0,
       lastAiMessage: null,
       deletedAt: null,
       createdAt: new Date(),