Improve erorr handling when cosigner key is missing

I recently deployed a new log for mtc_worker (#141) and forgot to create the corresponding secret (`SIGNING_KEY_shard2`). This is the error that I got:
```
POST https://bootstrap-mtca.cloudflareresearch.com/logs/shard2/add-entry - Ok @ 10/23/2025, 1:55:33 PM
POST http://fake_url.com/add_batch - Exception Thrown @ 10/23/2025, 1:55:34 PM
✘ [ERROR] Error: unreachable
```

That's not very helpful. Instead of panicking, we should handle this gracefully as a 5xx error.

Trace the /logs/{shard}/add-entry handler and the SIGNING_KEY_shard2 lookup, starting with the missing-secret failure path. Reproduce the deployment scenario with the cosigner key absent and verify that the endpoint returns a 5xx response instead of panicking or reporting an unhelpful error.

Written by the indexing model from the issue text.