feat: complete exam lifecycle — AI generation, submission, student session, and results

- Backend: AI generation fallbacks when OpenAI not configured, full exam
  submission saving all params (difficulty, rubric, entity, grading system,
  approval workflow) and creating linked question records per section
- Backend: new exam session controller with get_session, autosave, submit,
  status, and results endpoints; student attempt/answer/score models
- Backend: new controllers for entities, approval workflows, exam schedules
- Frontend: exam session split-layout with passage panel, question types
  (MCQ, T/F/NG, gap-fill, writing, speaking), timer, and review dialog
- Frontend: results page with percentage score, per-answer breakdown table
- Frontend: generation page dynamic dropdowns, full payload submission
- Frontend: updated types for ExamSessionSection, ExamQuestion options

Made-with: Cursor

custom_addons/encoach_ai/models/__init__.py

@@ -1,2 +1,3 @@
 from . import ai_settings
 from . import ai_log
+from . import constants

custom_addons/encoach_ai/models/constants.py

@@ -0,0 +1,13 @@
+CEFR_LEVELS = ['A1', 'A2', 'B1', 'B2', 'C1', 'C2']
+
+GPT_MODELS = {
+    'default': 'gpt-4o',
+    'fast': 'gpt-4o-mini',
+}
+
+TEMPERATURE = 0.7
+
+TOPICS = [
+    'Education', 'Technology', 'Health', 'Environment', 'Culture',
+    'Travel', 'Science', 'Business', 'Sports', 'Society',
+]