Replace print() with structured logging across backend

[BrandonS7]

What this does

The backend mixed print() statements with logging.getLogger() calls. For production on Cloud Run, structured logging is important for log aggregation, filtering by level, and debugging.

Replaced all print() calls in backend Python files with appropriate logger calls:

• Normal flow -> logger.info()
• Warnings -> logger.warning()
• Errors -> logger.error()

Added import logging and logger = logging.getLogger(__name__) to files that were missing it.

Files updated:

• backend/actions/process_slides.py
• backend/actions/transcribe_youtube.py
• backend/routes/voice_trace.py
• backend/services/langfuse_tracing.py

No behavior changes. No new dependencies.