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

backend/app/config.py

@@ -289,6 +289,13 @@ class Settings(BaseSettings):
         default="/data/log/bangui.log",
         description="Optional file path for writing application logs. Set to null to disable file logging.",
     )
+    suppress_third_party_logs: bool = Field(
+        default=True,
+        description=(
+            "When true, sets APScheduler and aiosqlite loggers to WARNING level. "
+            "Set to false to allow third-party libraries to emit DEBUG/INFO logs."
+        ),
+    )
     geoip_db_path: str | None = Field(
         default=None,
         description=(