Add Context APIs and refactor vocab models

Introduce Context and ContextGuide features: add Sequelize models (models/Context.js, models/ContextGuide.js), controllers (controllers/contextController.js, controllers/contextGuideController.js) and authenticated route handlers (routes/contextRoutes.js, routes/contextGuideRoutes.js). Wire the new routes into app.js and export the models from models/index.js. Refactor vocabulary: remove VocabForm, VocabMapping and VocabRelation models and relationships, update models/Vocab.js schema and indexes, and add migrate-vocab.js to drop/recreate the vocab table for the new schema. Also add a lesson editor UI (public/lesson-editor.html) and a small cleanup in models/Lesson.js.

models/Lesson.js

@@ -63,7 +63,6 @@ const Lesson = sequelize.define('lessons', {
     type: DataTypes.STRING(50),
     comment: 'Loại content cho URL: video, audio, pdf, external_link, youtube, etc.'
   },
-  
   duration_minutes: { 
     type: DataTypes.INTEGER,
     comment: 'Thời lượng (phút)'