fix: review fixes for MCP and API key feature

- Remove spurious ALTER TABLE client_targets DROP DEFAULT from migration
  (Prisma schema drift side-effect unrelated to this PR)
- Surface revoke errors in ApiKeysPage UI via deleteApiKey.isError
- Fix UpdateProjectInput.color type to allow null, removing unsafe cast
  in the update_project MCP tool handler

backend/prisma/migrations/20260316103132_add_api_keys/migration.sql

@@ -1,6 +1,3 @@
--- AlterTable
-ALTER TABLE "client_targets" ALTER COLUMN "working_days" DROP DEFAULT;
-
 -- CreateTable
 CREATE TABLE "api_keys" (
     "id" TEXT NOT NULL,