修复：软删除账号可复用用户名

backend/internal/database/migrations/009_users_username_active_unique.sql

@@ -0,0 +1,3 @@
+-- Soft-deleted accounts must not permanently reserve their login usernames.
+DROP INDEX IF EXISTS idx_users_username;
+CREATE UNIQUE INDEX idx_users_username ON users (username) WHERE deleted_at IS NULL;