Add Tencent Cloud ASR transcription provider

[jordansilly77-stack]

Summary

• Add a Tencent Cloud ASR provider using the official signed SentenceRecognition request flow
• Support local file base64 payloads, language/model aliases, optional Tencent ASR request flags, and structured response parsing
• Add mock-based tests for config, availability, signed payloads, response parsing, and error handling

Validation

• .venv/bin/python -m pytest tests/providers/test_tencentcloud.py tests/test_registry.py tests/test_cli.py -q
• .venv/bin/python -m compileall sapat tests/providers/test_tencentcloud.py

No secrets, media files, generated transcripts, or API keys are included.