feat: questions table with trilingual sentences

- status enum (draft/blocked/published), auto-timestamps
- sentence_de/en/se, blocked_topic
- Safe ALTER TABLE migration for existing placeholder
- /api/questions CRUD route

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/index.js

@@ -29,6 +29,7 @@ app.use('/api/words', auth, require('./routes/words'));
 app.use('/api/categories', auth, require('./routes/categories'));
 app.use('/api/objects', auth, require('./routes/objects'));
 app.use('/api/pairs', auth, require('./routes/pairs'));
+app.use('/api/questions', auth, require('./routes/questions'));
 
 // 404
 app.use((req, res) => {