Remove WebSocket streaming support from ASR backend

Description

The backend currently contains legacy WebSocket-based streaming infrastructure that is no longer required for the current REST-based ASR and diarization workflows.

This issue focuses on removing all unused streaming-related code, configurations, services, and tests to simplify the architecture, reduce maintenance overhead, and improve overall codebase stability.

Objectives

• Remove deprecated WebSocket streaming endpoints
• Delete unused streaming session management services
• Remove PCM16 streaming-specific transcription methods
• Clean up unused streaming/VAD configurations
• Remove obsolete tests and documentation references
• Ensure REST ASR and diarization flows remain fully functional

Scope of Work

API Cleanup

• Remove /stream WebSocket endpoint
• Remove WebSocket lifecycle/background cleanup logic

Service Cleanup

• Delete streaming_service.py

• Remove:

  • transcribe_pcm16
  • detect_language_pcm16

from ASR services and engines

Configuration Cleanup

Remove obsolete:

• STREAMING_*
• VAD_*

configuration values

Test Updates

• Remove streaming-specific test suites
• Update affected ASR/router/LID tests
• Maintain required test coverage

Expected Outcome

• Smaller and cleaner backend architecture
• Reduced technical debt
• Easier maintenance and debugging
• Faster test execution
• REST-only ASR pipeline with no unused streaming dependencies

Acceptance Criteria

• All WebSocket-related code removed
• All tests passing
• Coverage threshold maintained
• No impact on REST transcription or diarization APIs