feat: add configurable logging level for backend and Mini App

- Introduced a new `LOG_LEVEL` configuration option in the `.env.example` file to allow users to set the logging level (DEBUG, INFO, WARNING, ERROR).
- Updated the `Settings` class to include the `log_level` attribute, normalizing its value to ensure valid logging levels are used.
- Modified the logging setup in `run.py` to utilize the configured log level, enhancing flexibility in log management.
- Enhanced the Mini App to include the logging level in the JavaScript configuration, allowing for consistent logging behavior across the application.
- Added a new `logger.js` module for frontend logging, implementing level-based filtering and console delegation.
- Included unit tests for the new logger functionality to ensure proper behavior and level handling.

duty_teller/api/app.py

@@ -93,7 +93,11 @@ class NoCacheStaticMiddleware:
                         headers[idx] = (b"vary", existing + b", " + vary_val)
                 else:
                     headers.append((b"vary", vary_val))
-                message = {"type": "http.response.start", "status": message["status"], "headers": headers}
+                message = {
+                    "type": "http.response.start",
+                    "status": message["status"],
+                    "headers": headers,
+                }
             await send(message)
 
         await self.app(scope, receive, send_wrapper)
@@ -104,13 +108,16 @@ app.add_middleware(NoCacheStaticMiddleware)
 
 @app.get(
     "/app/config.js",
-    summary="Mini App config (language)",
-    description="Returns JS that sets window.__DT_LANG from DEFAULT_LANGUAGE. Loaded before main.js.",
+    summary="Mini App config (language, log level)",
+    description=(
+        "Returns JS that sets window.__DT_LANG and window.__DT_LOG_LEVEL. Loaded before main.js."
+    ),
 )
 def app_config_js() -> Response:
-    """Return JS assigning window.__DT_LANG for the webapp. No caching."""
+    """Return JS assigning window.__DT_LANG and window.__DT_LOG_LEVEL for the webapp. No caching."""
     lang = config.DEFAULT_LANGUAGE
-    body = f'window.__DT_LANG = "{lang}";'
+    log_level = config.LOG_LEVEL_STR.lower()
+    body = f'window.__DT_LANG = "{lang}";\nwindow.__DT_LOG_LEVEL = "{log_level}";'
     return Response(
         content=body,
         media_type="application/javascript; charset=utf-8",