fix(server): gracefully handle unknown jobs

[jrasm91]

Gracefully handle unknown jobs, by verifying there is a valid handler before calling it.

Sometimes there are jobs in redis when Immich is updated. In the case that job names have changed, it leads to errors like:

[Nest] 6  - 09/23/2024, 3:51:46 PM   ERROR [Microservices:JobService] Unable to run job handler (backgroundTask/undefined): TypeError: handler is not a function
[Nest] 6  - 09/23/2024, 3:51:46 PM   ERROR [Microservices:JobService] TypeError: handler is not a function
    at /usr/src/app/dist/services/job.service.js:148:42
    at Worker.workerHandler [as processFn] (/usr/src/app/dist/repositories/job.repository.js:90:46)
    at Worker.callProcessJob (/usr/src/app/node_modules/bullmq/dist/cjs/classes/worker.js:113:21)
    at Worker.processJob (/usr/src/app/node_modules/bullmq/dist/cjs/classes/worker.js:394:39)
    at /usr/src/app/node_modules/bullmq/dist/cjs/classes/worker.js:202:70
    at Worker.retryIfFailed (/usr/src/app/node_modules/bullmq/dist/cjs/classes/worker.js:581:30)
    at Worker.run (/usr/src/app/node_modules/bullmq/dist/cjs/classes/worker.js:202:45)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
[Nest] 6  - 09/23/2024, 3:51:46 PM   ERROR [Microservices:JobService] Object:
Error: Missing lock for job 28. failed
    at Scripts.finishedErrors (/usr/src/app/node_modules/bullmq/dist/cjs/classes/scripts.js:266:24)
    at Job.moveToFailed (/usr/src/app/node_modules/bullmq/dist/cjs/classes/job.js:427:32)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async handleFailed (/usr/src/app/node_modules/bullmq/dist/cjs/classes/worker.js:379:21)
    at async Worker.retryIfFailed (/usr/src/app/node_modules/bullmq/dist/cjs/classes/worker.js:581:24)
    ```