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/routes/pairs.js

@@ -0,0 +1,98 @@
+const router = require('express').Router();
+const { query } = require('../db');
+
+const STATUSES = ['draft', 'blocked', 'published'];
+const ANSWER_TYPES = ['yes_no', 'text', 'word'];
+
+const STATUS_TIMESTAMP = {
+  published: 'published_at',
+  blocked:   'blocked_at',
+};
+
+// GET /api/pairs
+router.get('/', async (req, res, next) => {
+  try {
+    const { status, answer_type, limit = 50, offset = 0 } = req.query;
+    const conditions = [];
+    const params = [Math.min(parseInt(limit), 500), parseInt(offset)];
+    if (status)      { params.push(status);      conditions.push(`status = $${params.length}`); }
+    if (answer_type) { params.push(answer_type); conditions.push(`answer_type = $${params.length}`); }
+    const where = conditions.length ? `WHERE ${conditions.join(' AND ')}` : '';
+    const result = await query(
+      `SELECT * FROM pairs ${where} ORDER BY created_at DESC LIMIT $1 OFFSET $2`,
+      params
+    );
+    res.json(result.rows);
+  } catch (err) { next(err); }
+});
+
+// GET /api/pairs/:id
+router.get('/:id', async (req, res, next) => {
+  try {
+    const result = await query('SELECT * FROM pairs WHERE id = $1', [req.params.id]);
+    if (!result.rows.length) return res.status(404).json({ error: 'Not found' });
+    res.json(result.rows[0]);
+  } catch (err) { next(err); }
+});
+
+// POST /api/pairs
+router.post('/', async (req, res, next) => {
+  try {
+    const { answer_type, difficulty_level, question_id,
+            positive_statement_id, negative_statement_id, blocked_topic } = req.body;
+
+    if (!answer_type || !ANSWER_TYPES.includes(answer_type))
+      return res.status(400).json({ error: `answer_type required, must be one of: ${ANSWER_TYPES.join(', ')}` });
+
+    const result = await query(
+      `INSERT INTO pairs
+         (answer_type, difficulty_level, question_id, positive_statement_id, negative_statement_id, blocked_topic)
+       VALUES ($1, $2, $3, $4, $5, $6) RETURNING *`,
+      [answer_type, difficulty_level || null, question_id || null,
+       positive_statement_id || null, negative_statement_id || null, blocked_topic || null]
+    );
+    res.status(201).json(result.rows[0]);
+  } catch (err) { next(err); }
+});
+
+// PATCH /api/pairs/:id
+router.patch('/:id', async (req, res, next) => {
+  try {
+    const allowed = ['status', 'answer_type', 'difficulty_level', 'blocked_topic',
+                     'question_id', 'positive_statement_id', 'negative_statement_id',
+                     'published_at', 'blocked_at'];
+    const fields = Object.keys(req.body).filter(k => allowed.includes(k));
+    if (!fields.length) return res.status(400).json({ error: 'No valid fields provided' });
+
+    if (req.body.status && !STATUSES.includes(req.body.status))
+      return res.status(400).json({ error: `status must be one of: ${STATUSES.join(', ')}` });
+    if (req.body.answer_type && !ANSWER_TYPES.includes(req.body.answer_type))
+      return res.status(400).json({ error: `answer_type must be one of: ${ANSWER_TYPES.join(', ')}` });
+
+    const tsField = STATUS_TIMESTAMP[req.body.status];
+    if (tsField && !req.body[tsField]) {
+      fields.push(tsField);
+      req.body[tsField] = new Date().toISOString();
+    }
+
+    const setClauses = fields.map((f, i) => `${f} = $${i + 1}`).join(', ');
+    const values = [...fields.map(f => req.body[f]), req.params.id];
+    const result = await query(
+      `UPDATE pairs SET ${setClauses} WHERE id = $${fields.length + 1} RETURNING *`,
+      values
+    );
+    if (!result.rows.length) return res.status(404).json({ error: 'Not found' });
+    res.json(result.rows[0]);
+  } catch (err) { next(err); }
+});
+
+// DELETE /api/pairs/:id
+router.delete('/:id', async (req, res, next) => {
+  try {
+    const result = await query('DELETE FROM pairs WHERE id = $1 RETURNING id', [req.params.id]);
+    if (!result.rows.length) return res.status(404).json({ error: 'Not found' });
+    res.status(204).end();
+  } catch (err) { next(err); }
+});
+
+module.exports = router;