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>

.gitignore

@@ -19,6 +19,7 @@ build/
 node_modules/
 .next/
 out/
+*.tsbuildinfo
 
 # 运行时数据
 runtime/
@@ -33,6 +34,7 @@ generated_images/
 # 数据目录中的运行时文件（保留源文件如 .tex）
 data/database/
 data/knowledge_base/
+data/images/
 
 # LaTeX 中间文件
 *.aux