fix(webapp): drop dead !backgroundWorker guard, comment unreachable throw

Both nits surfaced by a local code review of the previous commit:

- `findOrCreateBackgroundWorker` returns `Promise<BackgroundWorker>`
  (non-null). After `if (findOrCreateError) throw`, TypeScript narrows
  `backgroundWorker` to `BackgroundWorker` already, so the guard was dead
  code. Worse, returning silently there would have been a 200 with no body
  — strictly worse than the 5xx the rest of the file produces on internal
  errors.

- Annotate that `#failBackgroundWorkerDeployment` throws its own argument,
  so the outer `throw findOrCreateError` is only reachable on the non-SVE
  branch. Same control flow, less confusing for future readers.

Author: myftija

apps/webapp/app/v3/services/createDeploymentBackgroundWorkerV4.server.ts

@@ -109,15 +109,13 @@ export class CreateDeploymentBackgroundWorkerServiceV4 extends BaseService {
         // immediately instead of waiting 8 minutes for the timeout. Transient
         // races throw a plain `Error` and propagate as 5xx without failing.
         if (findOrCreateError instanceof ServiceValidationError) {
+          // `#failBackgroundWorkerDeployment` already throws its argument; the
+          // outer `throw` covers the non-SVE branch.
           await this.#failBackgroundWorkerDeployment(deployment, findOrCreateError);
         }
         throw findOrCreateError;
       }
 
-      if (!backgroundWorker) {
-        return;
-      }
-
       //upgrade the project to engine "V2" if it's not already
       if (environment.project.engine === "V1" && body.engine === "V2") {
         await this._prisma.project.update({