fix: add Migration 016 — playbooks.description column missing from DB schema

Playbook SQLAlchemy model has description column but production DB table
does not, causing seed_playbooks() to fail with UndefinedColumn error.
ADD COLUMN IF NOT EXISTS is idempotent — safe to re-run.

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

migrations/016_playbooks_description_column.sql

@@ -0,0 +1,11 @@
+-- Migration 016: Add missing description column to playbooks table
+-- Fixes: sqlalchemy.exc.ProgrammingError on PlayBook seed (description column missing)
+-- Author: Antigravity / ADR-013
+
+ALTER TABLE playbooks
+    ADD COLUMN IF NOT EXISTS description TEXT;
+
+DO $$
+BEGIN
+    RAISE NOTICE '✅ Migration 016 完成 — playbooks.description 欄位已補齊';
+END $$;