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/shared/i18n/messages/en/exam-homework.json

@@ -123,6 +123,57 @@
       "highErrorWarningDesc": "The following questions have an error rate above 70% and are recommended for focused review",
       "noData": "No analytics data yet. Data will be available after students submit and grading is complete.",
       "viewAnalytics": "View Analytics"
+    },
+    "richEditor": {
+      "title": "Rich Text Exam Builder",
+      "description": "Paste exam text, use toolbar to mark questions/groups/blanks/dotted chars, live preview on the right",
+      "basicInfo": "Basic Info",
+      "sourceText": "Source Text",
+      "sourceTextPlaceholder": "Paste the full exam text here...",
+      "aiAutoMark": "AI Auto-Mark",
+      "aiMarking": "AI marking...",
+      "aiMarkSuccess": "AI auto-marking complete",
+      "aiMarkFailed": "AI auto-marking failed",
+      "aiMarkHint": "After pasting exam text, click this button to let AI auto-detect question structure",
+      "editorArea": "Editor",
+      "previewArea": "Preview",
+      "emptyEditor": "Enter or paste exam content in the editor on the left",
+      "emptyPreview": "Preview will appear here",
+      "markQuestion": "Mark Question",
+      "markGroup": "Mark Group",
+      "markDotted": "Dotted Char",
+      "markBlank": "Blank",
+      "insertImage": "Insert Image",
+      "questionType": "Type",
+      "score": "Score",
+      "groupTitle": "Group Title",
+      "saveDraft": "Save Draft",
+      "saving": "Saving...",
+      "saveSuccess": "Exam draft created",
+      "saveFailed": "Failed to create",
+      "loadFormFailed": "Failed to load form data",
+      "titleRequired": "Please fill in the exam title",
+      "subjectGradeRequired": "Please select subject and grade",
+      "emptyContent": "Exam content is empty",
+      "pasteSourceFirst": "Please paste exam text first",
+      "bold": "Bold",
+      "italic": "Italic",
+      "strike": "Strikethrough",
+      "dotted": "Dotted",
+      "bulletList": "Bullet List",
+      "orderedList": "Ordered List",
+      "quote": "Quote",
+      "undo": "Undo",
+      "redo": "Redo",
+      "questionTypes": {
+        "single_choice": "Single Choice",
+        "multiple_choice": "Multiple Choice",
+        "true_false": "True/False",
+        "fill_blank": "Fill in the Blank",
+        "short_answer": "Short Answer",
+        "essay": "Essay",
+        "text": "Text"
+      }
     }
   },
   "homework": {
@@ -238,7 +289,18 @@
       "autoSaveSaved": "Auto-saved",
       "autoSaveError": "Auto-save failed. Will retry when network recovers.",
       "autoSaveRestored": "Restored unsaved answers from offline cache",
-      "autoSaveCacheError": "Failed to restore offline cache"
+      "autoSaveCacheError": "Failed to restore offline cache",
+      "scanTitle": "Answer Scan Upload",
+      "scanDescription": "After answering on paper, photograph and upload the full paper",
+      "selectImageFiles": "Please select image files",
+      "uploadFailed": "Upload failed",
+      "uploadSuccess": "Uploaded {{count}} images",
+      "deleteScan": "Delete",
+      "moveUp": "Move Up",
+      "moveDown": "Move Down",
+      "dragDropHint": "Drag images here, or click to select files",
+      "pageLabel": "Page {{page}}",
+      "scanDisabled": "Submitted, scan images cannot be modified"
     },
     "grade": {
       "title": "Grade",
@@ -286,7 +348,21 @@
       "batchSelectAtLeastOne": "Please select at least one submission",
       "batchFailed": "Batch grading failed",
       "selectAll": "Select All",
-      "selectRow": "Select this row"
+      "selectRow": "Select this row",
+      "scanGrading": "Scan Grading",
+      "prevSubmission": "Previous",
+      "nextSubmission": "Next",
+      "saveGrading": "Save Grading",
+      "questionsAndGrading": "Questions & Grading",
+      "studentScanImages": "Student Scan Images",
+      "loadingScans": "Loading...",
+      "loadingImages": "Loading scan images...",
+      "noScans": "No scan images",
+      "saveFailed": "Save failed",
+      "scanFeedbackPlaceholder": "Grading feedback (optional)...",
+      "scoreOutOf": "/ {{max}} pts",
+      "questionsCount": "Questions & Grading ({{count}} items)",
+      "scanPagesCount": "Student Scan Images ({{count}} pages)"
     },
     "review": {
       "title": "Review",