fix: route custom Loguru logs through Langflow logger

[erichare]

Summary

Fixes #12805.

Custom components that import logger directly from Loguru were bypassing Langflow's configured logging pipeline. As a result, messages such as logger.info(f"Processing {file_path}") could appear on stderr but not in the configured LANGFLOW_LOG_FILE, even when LANGFLOW_LOG_LEVEL=DEBUG and log retrieval/file logging were enabled.

This PR routes Loguru's default logger through Langflow's structlog configuration so those custom component messages reach the same sinks as the rest of Langflow logging. It also keeps repeated configure(...) calls from stacking duplicate rotating file handlers.

Root Cause

configure() configured Langflow's structlog logger and stdlib file handlers, but plain from loguru import logger usage kept Loguru's default sink. Custom component logs emitted through Loguru therefore skipped Langflow's configured file handler.

Changes

• Add a small Loguru bridge that forwards Loguru records into the configured structlog pipeline.
• Normalize invalid log levels before configuring Loguru, matching the existing structlog fallback behavior.
• Track and replace Langflow's rotating file handler on reconfiguration to avoid duplicate file writes.
• Add a regression test proving Loguru messages are written to the configured log file.

Validation

• uv run ruff check src/lfx/src/lfx/log/logger.py src/backend/tests/unit/test_logger.py
• uv run pytest src/backend/tests/unit/test_logger.py
• Manual reproduction of the issue pattern confirmed loguru.logger.info("Processing /tmp/example.png") lands in LANGFLOW_LOG_FILE.

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[Cristhianzl]

lgtm