feat: global exception handler, concurrent VLM, and docker security

- Add global FastAPI exception handler for unhandled errors
- Add get_current_user_obj() dependency for cleaner auth patterns
- Switch VLM image description from serial to asyncio.gather concurrency
- Extract score conversion to shared score_utils module
- Docker: use env vars for passwords, remove hardcoded API key default
- Add .gitattributes and update .gitignore for tar.gz and tsbuildinfo

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

backend/src/models/document.py

@@ -14,7 +14,7 @@ class Document(Base):
     
     id = Column(Integer, primary_key=True, index=True)
     user_id = Column(Integer, ForeignKey("users.id"), nullable=True)  # None表示系统文档
-    knowledge_base_id = Column(Integer, ForeignKey("knowledge_bases.id"), nullable=True)  # 所属知识库
+    knowledge_base_id = Column(Integer, ForeignKey("knowledge_bases.id"), nullable=True, index=True)
     filename = Column(String(255), nullable=False)
     original_filename = Column(String(255), nullable=False)
     file_path = Column(String(500), nullable=False)
@@ -47,7 +47,7 @@ class DocumentChunk(Base):
     __tablename__ = "document_chunks"
     
     id = Column(Integer, primary_key=True, index=True)
-    document_id = Column(Integer, ForeignKey("documents.id"), nullable=False)
+    document_id = Column(Integer, ForeignKey("documents.id"), nullable=False, index=True)
     chunk_index = Column(Integer, nullable=False)
     content = Column(Text, nullable=False)
     content_hash = Column(String(64), nullable=False)  # 内容哈希