fix(backend): split final UpdateAIJob + add goroutine recover() for AI job handlers

Split the single combined UpdateAIJob call at the end of each async
goroutine into two sequential calls (status first, payload second) so
that a failure writing the large results payload does not silently
prevent the job from being marked done. Added recover() to each
goroutine to catch and log panics. Affects chapter-names, description,
and image-gen handlers.

backend/internal/backend/handlers_image.go

@@ -559,6 +559,11 @@ func (s *Server) handleAdminImageGenAsync(w http.ResponseWriter, r *http.Request
 	go func() {
 		defer deregisterCancelJob(jobID)
 		defer jobCancel()
+		defer func() {
+			if r := recover(); r != nil {
+				logger.Error("admin: image-gen goroutine panic", "job_id", jobID, "recover", r)
+			}
+		}()
 
 		if jobCtx.Err() != nil {
 			_ = store.UpdateAIJob(context.Background(), jobID, map[string]any{
@@ -625,13 +630,19 @@ func (s *Server) handleAdminImageGenAsync(w http.ResponseWriter, r *http.Request
 			Guidance:    capturedReq.Guidance,
 		})
 
-		_ = store.UpdateAIJob(context.Background(), jobID, map[string]any{
-			"status":     string(domain.TaskStatusDone),
-			"items_done": 1,
+		if err := store.UpdateAIJob(context.Background(), jobID, map[string]any{
+			"status":      string(domain.TaskStatusDone),
+			"items_done":  1,
 			"items_total": 1,
-			"payload":    string(resultJSON),
-			"finished":   time.Now().Format(time.RFC3339),
-		})
+			"finished":    time.Now().Format(time.RFC3339),
+		}); err != nil {
+			logger.Error("admin: image-gen failed to mark job done", "job_id", jobID, "err", err)
+		}
+		if err := store.UpdateAIJob(context.Background(), jobID, map[string]any{
+			"payload": string(resultJSON),
+		}); err != nil {
+			logger.Error("admin: image-gen failed to write job payload", "job_id", jobID, "err", err)
+		}
 
 		logger.Info("admin: image-gen async done",
 			"job_id", jobID, "slug", capturedReq.Slug,

backend/internal/backend/handlers_textgen.go

@@ -516,6 +516,11 @@ func (s *Server) handleAdminTextGenChapterNamesAsync(w http.ResponseWriter, r *h
 	go func() {
 		defer deregisterCancelJob(jobID)
 		defer jobCancel()
+		defer func() {
+			if r := recover(); r != nil {
+				logger.Error("admin: text-gen chapter-names goroutine panic", "job_id", jobID, "recover", r)
+			}
+		}()
 
 		var allResults []proposedChapterTitle
 		chaptersDone := 0
@@ -574,12 +579,18 @@ func (s *Server) handleAdminTextGenChapterNamesAsync(w http.ResponseWriter, r *h
 		if jobCtx.Err() != nil {
 			status = domain.TaskStatusCancelled
 		}
-		_ = store.UpdateAIJob(context.Background(), jobID, map[string]any{
+		if err := store.UpdateAIJob(context.Background(), jobID, map[string]any{
 			"status":     string(status),
 			"items_done": chaptersDone,
 			"finished":   time.Now().Format(time.RFC3339),
-			"payload":    finalPayload,
-		})
+		}); err != nil {
+			logger.Error("admin: text-gen chapter-names failed to mark job done", "job_id", jobID, "err", err)
+		}
+		if err := store.UpdateAIJob(context.Background(), jobID, map[string]any{
+			"payload": finalPayload,
+		}); err != nil {
+			logger.Error("admin: text-gen chapter-names failed to write job payload", "job_id", jobID, "err", err)
+		}
 		logger.Info("admin: text-gen chapter-names async done",
 			"job_id", jobID, "slug", capturedSlug,
 			"results", len(allResults), "status", string(status))
@@ -902,6 +913,11 @@ func (s *Server) handleAdminTextGenDescriptionAsync(w http.ResponseWriter, r *ht
 	go func() {
 		defer deregisterCancelJob(jobID)
 		defer jobCancel()
+		defer func() {
+			if r := recover(); r != nil {
+				logger.Error("admin: text-gen description goroutine panic", "job_id", jobID, "recover", r)
+			}
+		}()
 
 		if jobCtx.Err() != nil {
 			_ = store.UpdateAIJob(context.Background(), jobID, map[string]any{
@@ -955,13 +971,19 @@ func (s *Server) handleAdminTextGenDescriptionAsync(w http.ResponseWriter, r *ht
 			NewDescription: strings.TrimSpace(newDesc),
 		})
 
-		_ = store.UpdateAIJob(context.Background(), jobID, map[string]any{
+		if err := store.UpdateAIJob(context.Background(), jobID, map[string]any{
 			"status":      string(domain.TaskStatusDone),
 			"items_done":  1,
 			"items_total": 1,
-			"payload":     string(resultJSON),
 			"finished":    time.Now().Format(time.RFC3339),
-		})
+		}); err != nil {
+			logger.Error("admin: text-gen description failed to mark job done", "job_id", jobID, "err", err)
+		}
+		if err := store.UpdateAIJob(context.Background(), jobID, map[string]any{
+			"payload": string(resultJSON),
+		}); err != nil {
+			logger.Error("admin: text-gen description failed to write job payload", "job_id", jobID, "err", err)
+		}
 		logger.Info("admin: text-gen description async done", "job_id", jobID, "slug", capturedMeta.Slug)
 	}()