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.

migrate-vocab.js

@@ -0,0 +1,36 @@
+/**
+ * Drop and recreate Vocab table with new schema
+ */
+const { sequelize } = require('./config/database');
+const { Vocab } = require('./models');
+
+async function migrateVocabTable() {
+  try {
+    console.log('🔄 Starting vocab table migration...');
+    
+    await sequelize.authenticate();
+    console.log('✅ Database connection OK');
+    
+    // Drop old vocab-related tables
+    console.log('🗑️  Dropping old vocab-related tables...');
+    await sequelize.query('DROP TABLE IF EXISTS `vocab_relation`');
+    await sequelize.query('DROP TABLE IF EXISTS `vocab_form`');
+    await sequelize.query('DROP TABLE IF EXISTS `vocab_mapping`');
+    await sequelize.query('DROP TABLE IF EXISTS `vocab`');
+    console.log('✅ Old tables dropped');
+    
+    // Recreate vocab table with new schema
+    console.log('📊 Creating new vocab table...');
+    await Vocab.sync({ force: true });
+    console.log('✅ Vocab table created with new schema');
+    
+    console.log('\n✅ Vocab migration complete!');
+    process.exit(0);
+  } catch (error) {
+    console.error('❌ Error migrating vocab table:', error.message);
+    console.error(error.stack);
+    process.exit(1);
+  }
+}
+
+migrateVocabTable();