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.

webapp/js/logger.js

@@ -0,0 +1,71 @@
+/**
+ * Frontend logger with configurable level (window.__DT_LOG_LEVEL).
+ * Only messages at or above the configured level are forwarded to console.
+ * Prefix [DutyTeller][level] for DevTools filtering.
+ */
+
+const LEVEL_ORDER = { debug: 0, info: 1, warn: 2, error: 3 };
+
+/**
+ * Resolve current log level from window.__DT_LOG_LEVEL. Default: info.
+ * @returns {string} One of "debug", "info", "warn", "error"
+ */
+function getLogLevel() {
+  const raw =
+    (typeof window !== "undefined" && window.__DT_LOG_LEVEL) || "info";
+  const level = String(raw).toLowerCase();
+  return LEVEL_ORDER.hasOwnProperty(level) ? level : "info";
+}
+
+/**
+ * Return true if message at level should be emitted (level >= configured).
+ * @param {string} messageLevel - "debug" | "info" | "warn" | "error"
+ * @returns {boolean}
+ */
+function shouldLog(messageLevel) {
+  const configured = getLogLevel();
+  const configuredNum = LEVEL_ORDER[configured] ?? 1;
+  const messageNum = LEVEL_ORDER[messageLevel] ?? 1;
+  return messageNum >= configuredNum;
+}
+
+const PREFIX = "[DutyTeller]";
+
+function logAt(level, args) {
+  if (!shouldLog(level)) return;
+  const consoleMethod =
+    level === "debug"
+      ? console.debug
+      : level === "info"
+        ? console.info
+        : level === "warn"
+          ? console.warn
+          : console.error;
+  const prefix = `${PREFIX}[${level}]`;
+  if (args.length === 0) {
+    consoleMethod(prefix);
+  } else if (args.length === 1) {
+    consoleMethod(prefix, args[0]);
+  } else {
+    consoleMethod(prefix, ...args);
+  }
+}
+
+/**
+ * Logger object with debug, info, warn, error (signature like console).
+ * Example: logger.info("Loaded", { count: 5 });
+ */
+export const logger = {
+  debug(msg, ...args) {
+    logAt("debug", [msg, ...args]);
+  },
+  info(msg, ...args) {
+    logAt("info", [msg, ...args]);
+  },
+  warn(msg, ...args) {
+    logAt("warn", [msg, ...args]);
+  },
+  error(msg, ...args) {
+    logAt("error", [msg, ...args]);
+  },
+};