Standardize logging config

[fahe1em1]

Summary

Closes #124

• Enhances healthchain/utils/logger.py with a centralized get_logger() function that configures a single console handler on the root healthchain logger
• All child loggers inherit this handler via standard library propagation — no per-module handler setup needed
• Log level is configurable via HEALTHCHAIN_LOG_LEVEL environment variable (defaults to INFO)
• Migrates all 50 modules from raw import logging + logging.getLogger(__name__) to the centralized get_logger(__name__)
• Existing add_handlers() function preserved for backward compatibility

Changes

• healthchain/utils/logger.py: Added get_logger(), _configure_root_healthchain_logger(), env var support
• healthchain/utils/__init__.py: Export get_logger
• healthchain/__init__.py: Use get_logger instead of manual handler setup
• 47 modules across interop/, sandbox/, gateway/, config/, pipeline/, models/, fhir/, io/: Replace import logging + logging.getLogger(__name__) with from healthchain.utils.logger import get_logger

Test plan

• All 719 tests pass (uv run pytest)
• Linter passes (uv run ruff check . --fix && uv run ruff format .)
• Verified HEALTHCHAIN_LOG_LEVEL=DEBUG correctly propagates to child loggers