server: Fix params leak between requests

[alubbe]

Example:

1. Request A sends bad audio, but also includes:

   -F carry_initial_prompt="true" \
   -F prompt="Always spell Kubernetes as K8s"

2. The server parses those fields into the shared params.

3. Then Request A fails while reading the audio and returns early.

4. Before the fix, the server never reached:

   params = default_params;

5. Request B sends a normal transcription request without any prompt.

6. The server could still use Request A’s stale settings, so Request B may unexpectedly behave as if it had:

   -F carry_initial_prompt="true" \
   -F prompt="Always spell Kubernetes as K8s"

That is confusing because Request B did nothing wrong, but its output can be influenced by a previous failed request. The fix makes every request start from a fresh local copy of the default params, so failed requests cannot contaminate later ones.