feat: pairs answer_type TEXT[], statements.answer bool

- pairs: answer_type changed from VARCHAR(20) to TEXT[] (multi-value)
  POST/PATCH now accept arrays like ['text','yes_no','word']
- statements: add answer BOOLEAN (nullable) for yes/no correct answer
- migration: ALTER TABLE pairs + ADD COLUMN statements.answer

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

src/db-migrate.js

@@ -319,6 +319,16 @@ async function migrate() {
     )
   `);
 
+  // pairs.answer_type → TEXT[] (multi-select)
+  await query(`ALTER TABLE pairs DROP CONSTRAINT IF EXISTS pairs_answer_type_check`).catch(() => {});
+  await query(`
+    ALTER TABLE pairs ALTER COLUMN answer_type TYPE TEXT[]
+    USING ARRAY[answer_type]::TEXT[]
+  `).catch(() => {});
+
+  // statements.answer — boolean nullable (for yes/no correct answer)
+  await query(`ALTER TABLE statements ADD COLUMN IF NOT EXISTS answer BOOLEAN`);
+
   // objects_created on pictures
   await query(`ALTER TABLE pictures ADD COLUMN IF NOT EXISTS objects_created BOOLEAN NOT NULL DEFAULT false`);
   await query(`ALTER TABLE pictures ADD COLUMN IF NOT EXISTS objects_created_at TIMESTAMPTZ`);