feat(VocabService): enhance logging and error handling in lesson assistant message flow

- Introduced detailed logging throughout the sendLessonAssistantMessage method to track request lifecycle, including start, abort conditions, and response handling.
- Improved error handling for various scenarios such as disabled assistant, unconfigured settings, empty messages, and fetch failures, providing clearer feedback to users.
- Added logging for response parsing and upstream errors to facilitate debugging and improve overall service reliability.

backend/services/vocabService.js

@@ -2808,17 +2808,33 @@ export default class VocabService {
   }
 
   async sendLessonAssistantMessage(hashedUserId, lessonId, payload = {}) {
+    const requestId = `assist-${Date.now().toString(36)}-${Math.random().toString(36).slice(2, 8)}`;
+    const log = (...args) => console.log(`[LLM ${requestId}]`, ...args);
+    const startedAt = Date.now();
+
     const user = await this._getUserByHashedId(hashedUserId);
     const lesson = await this.getLesson(hashedUserId, lessonId);
     const config = await this._getUserLlmConfig(user.id);
 
+    log('start', {
+      userId: user.id,
+      lessonId,
+      enabled: config.enabled,
+      configured: config.configured,
+      hasKey: config.hasKey,
+      baseUrl: config.baseUrl || '(default openai)',
+      model: config.model
+    });
+
     if (!config.enabled) {
+      log('aborted: assistant disabled in user settings');
       const err = new Error('Der Sprachassistent ist in deinen Einstellungen derzeit deaktiviert.');
       err.status = 400;
       throw err;
     }
 
     if (!config.configured) {
+      log('aborted: assistant not configured');
       const err = new Error('Der Sprachassistent ist noch nicht eingerichtet. Bitte hinterlege zuerst Modell und API-Zugang in den Einstellungen.');
       err.status = 400;
       throw err;
@@ -2826,6 +2842,7 @@ export default class VocabService {
 
     const message = String(payload?.message || '').trim();
     if (!message) {
+      log('aborted: empty message');
       const err = new Error('Bitte gib eine Nachricht für den Sprachassistenten ein.');
       err.status = 400;
       throw err;
@@ -2848,7 +2865,10 @@ export default class VocabService {
     const timeoutMs = Number.isFinite(configuredTimeout) && configuredTimeout >= 30000
       ? configuredTimeout
       : 300000;
-    const timeout = setTimeout(() => controller.abort(), timeoutMs);
+    const timeout = setTimeout(() => {
+      log('timeout reached, aborting fetch', { timeoutMs });
+      controller.abort();
+    }, timeoutMs);
 
     const temperatureByMode = {
       explain: 0.4,
@@ -2857,7 +2877,17 @@ export default class VocabService {
     };
     const temperature = Number.isFinite(temperatureByMode[mode]) ? temperatureByMode[mode] : 0.5;
 
+    log('request', {
+      endpoint,
+      mode,
+      temperature,
+      timeoutMs,
+      historyMessages: history.length,
+      messagePreview: message.slice(0, 120)
+    });
+
     let response;
+    let fetchStartedAt = Date.now();
     try {
       response = await fetch(endpoint, {
         method: 'POST',
@@ -2879,7 +2909,17 @@ export default class VocabService {
           ]
         })
       });
+      log('response received', {
+        status: response.status,
+        latencyMs: Date.now() - fetchStartedAt
+      });
     } catch (error) {
+      log('fetch failed', {
+        name: error?.name,
+        message: error?.message,
+        cause: error?.cause?.code || error?.cause?.errno || null,
+        latencyMs: Date.now() - fetchStartedAt
+      });
       const err = new Error(
         error?.name === 'AbortError'
           ? 'Der Sprachassistent hat das Antwort-Zeitlimit überschritten.'
@@ -2894,11 +2934,16 @@ export default class VocabService {
     let responseData = null;
     try {
       responseData = await response.json();
-    } catch {
+    } catch (parseError) {
+      log('failed to parse response JSON', { message: parseError?.message });
       responseData = null;
     }
 
     if (!response.ok) {
+      log('upstream returned non-ok', {
+        status: response.status,
+        body: responseData ? JSON.stringify(responseData).slice(0, 500) : null
+      });
       const messageFromApi = responseData?.error?.message || responseData?.message || 'Der Sprachassistent hat die Anfrage abgelehnt.';
       const err = new Error(messageFromApi);
       err.status = response.status || 502;
@@ -2907,11 +2952,20 @@ export default class VocabService {
 
     const reply = this._extractAssistantContent(responseData);
     if (!reply) {
+      log('empty reply from upstream', {
+        responsePreview: responseData ? JSON.stringify(responseData).slice(0, 500) : null
+      });
       const err = new Error('Der Sprachassistent hat keine verwertbare Antwort geliefert.');
       err.status = 502;
       throw err;
     }
 
+    log('done', {
+      totalMs: Date.now() - startedAt,
+      model: responseData?.model || config.model,
+      replyLength: reply.length
+    });
+
     return {
       reply,
       model: responseData?.model || config.model,