feat: pairs table with questions/statements placeholders

- pairs: status, answer_type enum (yes_no/text/word), difficulty_level,
  FK to questions + 2x statements (positive/negative), auto-timestamps
- questions + statements placeholder tables for future use
- Safe ALTER TABLE migration for existing pairs placeholder
- /api/pairs CRUD route, answer_type required on create

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

src/index.js

@@ -28,6 +28,7 @@ app.use('/api/pictures', auth, require('./routes/pictures'));
 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'));
 
 // 404
 app.use((req, res) => {