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/Vocab.js

@@ -8,21 +8,32 @@ const Vocab = sequelize.define('Vocab', {
       primaryKey: true,
       comment: 'Unique identifier for vocabulary entry'
     },
-    vocab_code: {
-      type: DataTypes.STRING(50),
-      unique: true,
-      allowNull: false,
-      comment: 'Unique code for vocabulary (e.g., vocab-001-eat)'
+    // Từ thực tế (wash, washes, washing, ate, eaten...)
+    text: { 
+      type: DataTypes.STRING(100), 
+      allowNull: false, 
+      index: true 
     },
-    base_word: {
-      type: DataTypes.STRING(100),
-      allowNull: false,
-      comment: 'Base form of the word'
+    // Từ gốc để nhóm lại (wash, eat...)
+    base_word: { 
+      type: DataTypes.STRING(100), 
+      allowNull: false, 
+      index: true 
     },
+    // Đã xuất hiện trong khối nào, bài học nào, lesson nào
+    // Ví dụ 111 là grade 1, unit 1, lesson 1
+    location: {
+      type: DataTypes.INTEGER,
+      comment: 'Location or source of the vocabulary'
+    },
+    // Loại biến thể (V1, V2, V3, V_ing, Noun_Form...)
+    form_key: { 
+      type: DataTypes.STRING(20), 
+      defaultValue: 'base' 
+    },
+    // Nội dung dùng chung (có thể lưu JSON để dễ quản lý hoặc dùng chung cho cả group)
     translation: {
-      type: DataTypes.STRING(200),
-      allowNull: false,
-      comment: 'Vietnamese translation'
+      type: DataTypes.STRING(200)
     },
     difficulty_score: {
       type: DataTypes.INTEGER,
@@ -33,6 +44,10 @@ const Vocab = sequelize.define('Vocab', {
       type: DataTypes.STRING(100),
       comment: 'Category of the word (e.g., Action Verbs, Nouns)'
     },
+    topic: {
+      type: DataTypes.STRING(100),
+      comment: 'Topic of the word (e.g., Food, Travel, Education)'
+    },
     images: {
       type: DataTypes.JSON,
       comment: 'Array of image URLs'
@@ -73,8 +88,8 @@ const Vocab = sequelize.define('Vocab', {
     updatedAt: 'updated_at',
     indexes: [
       {
-        name: 'idx_vocab_code',
-        fields: ['vocab_code']
+        name: 'idx_vocab_text',
+        fields: ['text']
       },
       {
         name: 'idx_base_word',
@@ -83,6 +98,10 @@ const Vocab = sequelize.define('Vocab', {
       {
         name: 'idx_category',
         fields: ['category']
+      },
+      {
+        name: 'idx_location',
+        fields: ['location']
       }
     ]
   });