feat: multimodal RAG with PDF image extraction and display

Extract images from PDFs using pymupdf, generate descriptions via
Qwen3-VL-8B, store in ChromaDB alongside text chunks, and render
images in chat answers. Includes image proxy rewrite, force re-process
endpoint, and VLM API timeout.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/rag/document_loaders.py

@@ -1,8 +1,9 @@
 """
-LangChain 1.0 文档加载器封装
+LangChain 1.0 文档加载器封装 + PDF图片提取
 """
-from typing import List, Optional
+from typing import List, Optional, Dict
 from pathlib import Path
+import fitz  # pymupdf
 from langchain_community.document_loaders import (
     PyPDFLoader,
     Docx2txtLoader,
@@ -11,6 +12,60 @@ from langchain_community.document_loaders import (
 )
 from langchain_core.documents import Document
 
+
+class PDFImageExtractor:
+    """使用pymupdf从PDF中提取内嵌图片"""
+
+    @staticmethod
+    def extract_images(file_path: str, output_dir: str) -> List[dict]:
+        """提取PDF中所有图片，返回图片元数据列表"""
+        Path(output_dir).mkdir(parents=True, exist_ok=True)
+        images = []
+        doc = fitz.open(file_path)
+
+        for page_num in range(len(doc)):
+            page = doc[page_num]
+            # 获取页面文本作为图片上下文
+            page_text = page.get_text("text")
+            # 提取页面内嵌图片
+            image_list = page.get_images(full=True)
+
+            for img_idx, img_info in enumerate(image_list):
+                xref = img_info[0]
+                try:
+                    base_image = doc.extract_image(xref)
+                    image_bytes = base_image["image"]
+                    ext = base_image["ext"]
+
+                    # 过滤太小的图片（图标、装饰元素等）
+                    if len(image_bytes) < 2048:
+                        continue
+
+                    # 图片周围文本（取该页文字前后各300字作为上下文）
+                    context_start = max(0, page_text.find(
+                        page_text[:len(page_text)//2]) if len(page_text) > 600
+                        else 0)
+                    context_text = page_text[context_start:context_start+600].strip()
+
+                    filename = f"page{page_num+1}_img{img_idx+1}.{ext}"
+                    output_path = Path(output_dir) / filename
+                    output_path.write_bytes(image_bytes)
+
+                    images.append({
+                        "path": str(output_path),
+                        "filename": filename,
+                        "page": page_num + 1,
+                        "context_text": context_text,
+                        "size": len(image_bytes),
+                    })
+                except Exception as e:
+                    print(f"[ImageExtractor] 提取图片失败 page={page_num+1} img={img_idx}: {e}")
+                    continue
+
+        doc.close()
+        return images
+
+
 class DocumentLoaderFactory:
     """文档加载器工厂"""