Ship legacy migrate APIs, portfolio/blog old-id schema, and admin migrate/purge flows.

Co-authored-by: Cursor <cursoragent@cursor.com>

database/migrations/046_blogs_old_id.sql

@@ -0,0 +1,7 @@
+-- Link migrated blogs to legacy WillaEngine article ids
+ALTER TABLE blogs
+  ADD COLUMN IF NOT EXISTS old_id BIGINT;
+
+CREATE UNIQUE INDEX IF NOT EXISTS blogs_business_old_id_unique
+  ON blogs (business_id, old_id)
+  WHERE old_id IS NOT NULL;