♻️ refactor: rewrite OAuth provision business logic for new user schema

prisma/migrations/20260528141807_increate_limit_oauth_token/migration.sql

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+-- DropForeignKey
+ALTER TABLE "user_preferences" DROP CONSTRAINT "user_preferences_char_as_partner_id_fkey";
+
+-- AlterTable
+ALTER TABLE "user_oauth_accounts" ALTER COLUMN "provider_token" SET DATA TYPE TEXT,
+ALTER COLUMN "refresh_token" SET DATA TYPE TEXT;
+
+-- AlterTable
+ALTER TABLE "user_preferences" ALTER COLUMN "char_as_partner_id" DROP NOT NULL;
+
+-- AddForeignKey
+ALTER TABLE "user_preferences" ADD CONSTRAINT "user_preferences_char_as_partner_id_fkey" FOREIGN KEY ("char_as_partner_id") REFERENCES "characters"("id") ON DELETE SET NULL ON UPDATE CASCADE;