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/db/src/schema/diagram.ts

@@ -48,6 +48,7 @@ export const diagram = pgTable("diagram", {
     .references(() => user.id, { onDelete: "cascade" })
     .notNull(),
   projectId: text(),
+  sortOrder: integer().default(0),
   lastAiMessage: text(),
   deletedAt: timestamp(),
   createdAt: timestamp().defaultNow(),