refactor: answer_type single TEXT + new 'question' type

- Convert TEXT[] back to TEXT (take first element of existing arrays)
- Valid values: yes_no, text, question, word
- API validation updated for single string

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

src/db-migrate.js

@@ -319,11 +319,18 @@ async function migrate() {
     )
   `);
 
-  // pairs.answer_type → TEXT[] (multi-select)
+  // pairs.answer_type → single TEXT (was TEXT[], now back to single value + new 'question' type)
   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[]
+    ALTER TABLE pairs ALTER COLUMN answer_type TYPE TEXT
+    USING (CASE
+      WHEN answer_type IS NULL OR array_length(answer_type::TEXT[], 1) IS NULL THEN 'text'
+      ELSE (answer_type::TEXT[])[1]
+    END)
+  `).catch(() => {});
+  await query(`
+    ALTER TABLE pairs ADD CONSTRAINT pairs_answer_type_check
+    CHECK (answer_type IN ('yes_no', 'text', 'question', 'word'))
   `).catch(() => {});
 
   // statements.answer — boolean nullable (for yes/no correct answer)