fix: resolve P0/P1 code quality issues

P0 fixes:
- ModelError.Is(): use exact matching instead of substring contains()
- shouldClearStickySession: add context param for cancellation/tracing

P1 fixes:
- TODO stubs: return 501 Not Implemented errors
- validateInstanceSignature: deduplicate to shared validateCodeSignature()
- Error messages: standardize to English only
- http.go: remove pseudo if-else with duplicate branches

.gitattributes

@@ -0,0 +1,22 @@
+# 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
+
+# 前端 源代码文件
+*.ts text eol=lf
+*.tsx text eol=lf
+*.js text eol=lf
+*.jsx text eol=lf
+*.vue 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