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/api.js

@@ -6,6 +6,7 @@ import { FETCH_TIMEOUT_MS } from "./constants.js";
 import { getInitData } from "./auth.js";
 import { state } from "./dom.js";
 import { t } from "./i18n.js";
+import { logger } from "./logger.js";
 
 /**
  * Build fetch options with init data header, Accept-Language and timeout abort.
@@ -49,10 +50,17 @@ export async function apiGet(path, params = {}, options = {}) {
   const query = new URLSearchParams(params).toString();
   const url = query ? `${base}${path}?${query}` : `${base}${path}`;
   const initData = getInitData();
+  logger.debug("API request", path, params);
   const opts = buildFetchOptions(initData, options.signal);
   try {
     const res = await fetch(url, { headers: opts.headers, signal: opts.signal });
+    if (!res.ok) {
+      logger.warn("API non-OK response", path, res.status);
+    }
     return res;
+  } catch (e) {
+    logger.error("API request failed", path, e);
+    throw e;
   } finally {
     opts.cleanup();
   }
@@ -69,6 +77,7 @@ export async function apiGet(path, params = {}, options = {}) {
 export async function fetchDuties(from, to, signal) {
   const res = await apiGet("/api/duties", { from, to }, { signal });
   if (res.status === 403) {
+    logger.warn("Access denied", from, to);
     let detail = t(state.lang, "access_denied");
     try {
       const body = await res.json();