refactor(logging): replace structlog with stdlib logging compat layer

- Remove structlog dependency from backend/pyproject.toml
- Add app.utils.logging_compat shim for keyword-arg logging API
- Add app.utils.json_formatter for JSON log output with extra fields
- Update all backend modules to use logging_compat.get_logger()
- Update docstrings in log_sanitizer.py and json_formatter.py
- Update test comment in test_async_utils.py
- Record 406 failing tests in Docs/Tasks.md for tracking

Docs/TROUBLESHOOTING.md

@@ -418,6 +418,65 @@ Then set it in your `.env` file or environment variables.
 
 ---
 
+## Enabling Debug Logs for Third-Party Libraries
+
+BanGUI suppresses verbose DEBUG logs from APScheduler and aiosqlite by default (see `Docs/Observability.md`). When troubleshooting scheduler or database issues, you can temporarily re-enable these logs.
+
+### Quick method (environment variable)
+
+Set `BANGUI_SUPPRESS_THIRD_PARTY_LOGS=false` and ensure `BANGUI_LOG_LEVEL=debug`:
+
+```bash
+BANGUI_SUPPRESS_THIRD_PARTY_LOGS=false \
+BANGUI_LOG_LEVEL=debug \
+python -m uvicorn app.main:create_app
+```
+
+This allows APScheduler and aiosqlite to inherit the application log level without editing code.
+
+### Code method (for permanent changes)
+
+If you need to change the level for a specific library only, edit `backend/app/main.py` inside `_configure_logging()`:
+
+```python
+logging.getLogger("apscheduler").setLevel(logging.DEBUG)
+```
+
+Restart the application. You will see scheduler polling messages such as:
+- `Looking for jobs to run`
+- `Next wakeup is due at ...`
+- `Running job ...`
+
+### Reverting
+
+Remove the environment variable or code change and restart. When suppression is re-enabled, the loggers return to `WARNING` level.
+
+---
+
+## Plain Text Logs Still Appearing
+
+If `bangui.log` contains plain text lines that are not JSON, a library is bypassing structlog's `ProcessorFormatter`.
+
+**Diagnosis:**
+
+1. Identify the logger name in the plain text line (usually at the start of the line).
+2. Check whether the logger is listed in `backend/app/main.py::_configure_logging()` under the third-party overrides.
+3. Verify that `structlog.stdlib.ProcessorFormatter` is attached to all handlers:
+   ```python
+   for handler in handlers:
+       handler.setFormatter(formatter)
+   ```
+
+**Common causes:**
+
+| Cause | Fix |
+|-------|-----|
+| Library initializes its own handler after startup | Add `logging.getLogger("library_name").setLevel(logging.WARNING)` in `_configure_logging()`. |
+| Custom handler added outside `_configure_logging()` | Ensure all handlers use `structlog.stdlib.ProcessorFormatter`. |
+| Log emitted before `_configure_logging()` is called | Move logging configuration earlier in the lifespan or app factory. |
+
+---
+
 ## Getting Help
 
 If issues persist after following this guide: