fix: 使用容器内可用字体生成验证码

尝试多个字体路径，优先使用Liberation字体

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app.py

@@ -1065,9 +1065,19 @@ def generate_captcha():
             draw.point((x, y), fill=(random.randint(0, 200), random.randint(0, 200), random.randint(0, 200)))
 
         # 绘制验证码文字 - 增大字体
+        font = None
+        font_paths = [
+            "/usr/share/fonts/truetype/liberation/LiberationSans-Bold.ttf",
+            "/usr/share/fonts/truetype/dejavu/DejaVuSans-Bold.ttf",
+            "/usr/share/fonts/truetype/freefont/FreeSansBold.ttf",
+        ]
+        for font_path in font_paths:
             try:
-            font = ImageFont.truetype("/usr/share/fonts/truetype/dejavu/DejaVuSans-Bold.ttf", 42)
+                font = ImageFont.truetype(font_path, 42)
+                break
             except:
+                continue
+        if font is None:
             font = ImageFont.load_default()
 
         for i, char in enumerate(code):