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/ContextGuide.js

@@ -0,0 +1,51 @@
+const { DataTypes } = require('sequelize');
+const { sequelize } = require('../config/database');
+
+const ContextGuide = sequelize.define('ContextGuide', {
+  uuid: {
+    type: DataTypes.UUID,
+    defaultValue: DataTypes.UUIDV4,
+    primaryKey: true,
+    comment: 'Unique identifier for context guide'
+  },
+  title: {
+    type: DataTypes.STRING(255),
+    allowNull: false,
+    comment: 'Guide title'
+  },
+  name: {
+    type: DataTypes.STRING(255),
+    allowNull: false,
+    comment: 'Target field name for AI processing'
+  },
+  data: {
+    type: DataTypes.JSON,
+    allowNull: false,
+    comment: 'Guide data (text or json)'
+  },
+  created_at: {
+    type: DataTypes.DATE,
+    defaultValue: DataTypes.NOW
+  },
+  updated_at: {
+    type: DataTypes.DATE,
+    defaultValue: DataTypes.NOW
+  }
+}, {
+  tableName: 'context_guide',
+  timestamps: true,
+  createdAt: 'created_at',
+  updatedAt: 'updated_at',
+  indexes: [
+    {
+      name: 'idx_context_guide_title',
+      fields: ['title']
+    },
+    {
+      name: 'idx_context_guide_name',
+      fields: ['name']
+    }
+  ]
+});
+
+module.exports = ContextGuide;