feat(exams,homework): add rich text exam editor and scan-based grading

- Add Tiptap-based rich text editor with custom extensions (dotted-mark,
  blank-node, image-node, group-block, question-block) for exam creation
- Add AI auto-marking action to convert pasted exam text to structured editor doc
- Add resizable split-panel layout for editor + live preview
- Add student scan upload (photo of paper answers) with drag-drop and reorder
- Add scan image viewer with zoom/rotate/fullscreen for teachers
- Add scan grading view with side-by-side questions and scan images
- Add /teacher/exams/new and /teacher/homework/submissions/[id]/scan-grading routes
- Fix getScansAction to support both teacher (HOMEWORK_GRADE) and student
  (HOMEWORK_SUBMIT) permission scopes
- Add i18n keys for rich editor, scan upload, and scan grading (zh-CN/en)
- Sync architecture diagrams (004/005) with new modules, routes, and deps

src/modules/files/types.ts

@@ -1,5 +1,5 @@
 // 文件关联的目标资源类型（多态关联）
-export type FileTargetType = "exam" | "textbook" | "question" | "announcement"
+export type FileTargetType = "exam" | "textbook" | "question" | "announcement" | "homework"
 
 // 文件附件记录（DB 行的 TypeScript 表示）
 export interface FileAttachment {