chore: add .gitattributes to enforce LF line endings

Ensures consistent line endings for SQL migration files, Go source,
shell scripts, YAML configs, and Dockerfiles. Fixes checksum mismatches
on Windows where CRLF line endings cause migration hash differences.

.gitattributes

+# 确保所有 SQL 迁移文件使用 LF 换行符
+backend/migrations/*.sql text eol=lf
+
+# Go 源代码文件
+*.go text eol=lf
+
+# Shell 脚本
+*.sh text eol=lf
+
+# YAML/YML 配置文件
+*.yaml text eol=lf
+*.yml text eol=lf
+
+# Dockerfile
+Dockerfile text eol=lf