debug: add watermark debug logging

app/services/pdf_generator.py

@@ -272,7 +272,10 @@ def generate_pdf(plan_id, student_name, content, output_dir, rendered_report=Non
 
     # 水印函数（每页都绘制）
     def draw_watermark(c, doc):
-        if not watermark_text or not CHINESE_FONT_OK:
+        if not watermark_text:
+            return
+        if not CHINESE_FONT_OK:
+            print(f"[WATERMARK] Skipping watermark - CHINESE_FONT_OK=False, FONT_PATH={FONT_PATH}")
             return
         c.saveState()
         try:
@@ -284,8 +287,9 @@ def generate_pdf(plan_id, student_name, content, output_dir, rendered_report=Non
             c.rotate(45)
             # 绘制水印文字（居中）
             c.drawCentredString(0, 0, watermark_text)
-        except Exception:
-            pass  # 字体问题则跳过水印
+            print(f"[WATERMARK] Drawn: {watermark_text}")
+        except Exception as e:
+            print(f"[WATERMARK] Error: {e}")
         c.restoreState()
 
     doc.build(pdf.elements, onFirstPage=draw_watermark, onLaterPages=draw_watermark)