update voxcpm docs

[minhsaco99]

Description

Updates README.md and docs/api.md to document the new VoxCPM TTS engine and reflect
the current API state. Previously, documentation examples referenced the unimplemented
coqui engine, which could confuse users. This update:

• Replaces engine=coqui examples with working engine=voxcpm examples
• Adds VoxCPM to the supported engines table with "Ready" status
• Documents VoxCPM-specific parameters for voice cloning (prompt_wav_path,
  prompt_text)

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• New Engine (STT/TTS provider)
• Breaking change (fix or feature that would cause existing functionality to not
  work as expected)
• Refactor (non-breaking code cleanup or optimization)
• Documentation update
• Performance improvement

Checklist

• I have read the CONTRIBUTING guide
• My code follows the project's code style (make format)
• Linting passes (make lint)
• Tests pass (make test)
• Documentation updated (if needed)
• No sensitive information (API keys, secrets) included

Related Issues

Closes #13

Testing & Verification

Automated Tests

• Unit tests added/updated
• All existing tests pass

Manual Verification (if applicable)

• Verified markdown renders correctly
• Confirmed curl examples match actual API parameters
• Cross-referenced VoxCPM features with app/engines/tts/voxcpm/engine.py

API Endpoints Tested (if applicable)

• Batch endpoint (POST /api/v1/stt/transcribe or /tts/synthesize)
• SSE streaming (POST .../stream)
• WebSocket (WS .../ws)

Engine-Specific Tests (if applicable)

N/A - Documentation only

Security Impact

• No security implications
• Security impact (please describe below)