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/Observability.md

@@ -98,6 +98,44 @@ log.error("fail2ban_start_failed", stdout=stdout_raw, stderr=stderr_raw)  # Neve
 
 ---
 
+## Third-Party Library Logs
+
+BanGUI uses **structlog** for all application logs, but third-party libraries often emit plain text through Python's standard `logging` module. To maintain uniform JSON output and reduce noise, the following libraries have their log levels overridden to `WARNING`:
+
+| Library | Logger Name | Level | Rationale |
+|---------|-------------|-------|-----------|
+| APScheduler | `apscheduler` | `WARNING` | Suppresses routine scheduler polling ("Looking for jobs to run", "Next wakeup is due at...") while preserving job failure warnings. |
+| aiosqlite | `aiosqlite` | `WARNING` | Suppresses database operation traces and connection details while preserving connection errors. |
+
+These overrides are applied in `backend/app/main.py::_configure_logging()` immediately after `logging.basicConfig()`.
+
+### Disabling Suppression
+
+Set the environment variable `BANGUI_SUPPRESS_THIRD_PARTY_LOGS=false` to allow APScheduler and aiosqlite to emit their normal DEBUG/INFO logs. This is useful when troubleshooting scheduler or database issues in development.
+
+```bash
+BANGUI_SUPPRESS_THIRD_PARTY_LOGS=false python -m uvicorn app.main:create_app
+```
+
+When suppression is disabled, the loggers inherit the application's `BANGUI_LOG_LEVEL` (e.g., `debug`).
+
+### Uniform JSON Formatting
+
+All stdlib logs — including those from third-party libraries — are intercepted by `structlog.stdlib.ProcessorFormatter` and rendered as JSON. This ensures every log line in `bangui.log` is machine-readable, regardless of its source.
+
+### Adding New Overrides
+
+When integrating a new library that emits verbose DEBUG logs:
+
+```python
+# In backend/app/main.py, inside _configure_logging()
+logging.getLogger("new_library").setLevel(logging.WARNING)
+```
+
+Use `WARNING` as the default to still capture errors and warnings. Only use `ERROR` if the library is exceptionally noisy and its warnings are not actionable.
+
+---
+
 ## Structured Logging Best Practices
 
 ### Log Levels