feat(server): Improve HTTP Connection Abort Handling

[sachaarbonel]

Changes

• Replaced static abort flags with direct HTTP connection state checking
• Added proper handling of client disconnections during audio processing
• Implemented HTTP status code 499 (Client Closed Request) for client disconnects
• Added HTTP status code 500 for internal server errors

Technical Details

• Removed redundant encoder_begin_callback and simplified abort mechanism
• Added direct integration with httplib's connection state tracking
• Improved error handling and status code reporting
• Eliminated potential data races from static abort flags

Impact

This change improves the server's ability to handle client disconnections gracefully and provides more accurate HTTP status codes for different error scenarios.