fix: ElevenLabs voice_id is hardcoded, add ELEVENLABS_VOICE_ID and ELEVENLABS_LANGUAGE to settings

## Bug Description

`application/tts/elevenlabs.py` hardcodes the ElevenLabs voice ID with no way to change it without editing source code:

```python
voice_id="nPczCjzI2devNBz1zQrb", # hardcoded, line 19
```

There is no `ELEVENLABS_VOICE_ID` key in `application/core/settings.py`. The local `lang = "en"` variable is also dead — assigned but only returned, never used to configure synthesis language. Every other ElevenLabs setting (`ELEVENLABS_API_KEY`) is env-configurable. The voice ID is the only exception.

## Expected Behavior

Operators should be able to set `ELEVENLABS_VOICE_ID` and `ELEVENLABS_LANGUAGE` via environment variables, consistent with how all other provider settings work.

## Proposed Fix

Add `ELEVENLABS_VOICE_ID: str = "nPczCjzI2devNBz1zQrb"` and `ELEVENLABS_LANGUAGE: str = "en"` to `Settings` (keeping existing values as defaults for backward compatibility) and update `elevenlabs.py` to read from settings.

Start with application/core/settings.py to see how provider settings are declared, then read application/tts/elevenlabs.py and trace the current voice ID and language values. Confirm that both environment variables use the stated backward-compatible defaults and that the ElevenLabs synthesis call reads them; run the relevant existing project tests if available.

Written by the indexing model from the issue text.