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.

app.js

@@ -40,6 +40,8 @@ const grammarRoutes = require('./routes/grammarRoutes');
 const storyRoutes = require('./routes/storyRoutes');
 const learningContentRoutes = require('./routes/learningContentRoutes');
 const uploadRoutes = require('./routes/uploadRoutes');
+const contextRoutes = require('./routes/contextRoutes');
+const contextGuideRoutes = require('./routes/contextGuideRoutes');
 
 /**
  * Initialize Express Application
@@ -165,6 +167,8 @@ app.get('/api', (req, res) => {
       games: '/api/games',
       gameTypes: '/api/game-types',
       vocab: '/api/vocab',
+      contexts: '/api/contexts',
+      contextGuides: '/api/context-guides',
       upload: '/api/upload',
     },
     documentation: '/api-docs',
@@ -224,6 +228,8 @@ app.use('/api/grammar', grammarRoutes);
 app.use('/api/stories', storyRoutes);
 app.use('/api/learning-content', learningContentRoutes);
 app.use('/api/upload', uploadRoutes);
+app.use('/api/contexts', contextRoutes);
+app.use('/api/context-guides', contextGuideRoutes);
 
 /**
  * Queue Status Endpoint