feat: 配置 golangci-lint 静态分析并修复存量违规

- 新增 backend/.golangci.yml 配置 12 个 linter（forbidigo、errorlint、errcheck、staticcheck、revive、gocritic、gosec、bodyclose、noctx、nilerr、goimports、gocyclo）
- 新增 lefthook.yml 配置 pre-commit hook 自动运行 lint
- 修复存量代码违规：errors.Is/As 替换、zap.Error 替换、import 排序、errcheck 修复
- 更新 README 补充编码规范说明
- 归档 backend-code-lint 变更

backend/.golangci.yml

@@ -0,0 +1,91 @@
+run:
+  timeout: 5m
+  tests: true
+
+linters:
+  disable-all: true
+  enable:
+    - forbidigo
+    - errorlint
+    - errcheck
+    - staticcheck
+    - revive
+    - gocritic
+    - gosec
+    - bodyclose
+    - noctx
+    - nilerr
+    - goimports
+    - gocyclo
+
+linters-settings:
+  errcheck:
+    check-blank: true
+    check-type-assertions: true
+    exclude-functions:
+      - fmt.Fprintf
+  forbidigo:
+    analyze-types: true
+    forbid:
+      - p: '^fmt\.Print.*$'
+        msg: 使用 zap logger，不要直接输出到 stdout/stderr
+      - p: '^fmt\.Fprint.*$'
+        msg: 使用 zap logger，不要直接输出到 stdout/stderr
+      - p: '^log\.(Print|Println|Printf|Fatal|Fatalln|Fatalf|Panic|Panicln|Panicf)$'
+        msg: 使用 zap logger，不要使用标准库 log
+      - p: '^zap\.L$'
+        msg: 通过依赖注入传递 *zap.Logger，不要使用全局 logger
+      - p: '^zap\.S$'
+        msg: 不使用 Sugar logger
+  revive:
+    rules:
+      - name: exported
+      - name: var-naming
+      - name: indent-error-flow
+      - name: error-strings
+      - name: error-return
+      - name: blank-imports
+      - name: context-as-argument
+      - name: unexported-return
+  goimports:
+    local-prefixes: nex/backend
+  gocyclo:
+    min-complexity: 10
+
+issues:
+  exclude-dirs:
+    - tests/mocks
+  exclude-generated: true
+  exclude-rules:
+    - path: '(_test\.go|tests/)'
+      linters:
+        - forbidigo
+    - path: '(_test\.go|tests/)'
+      linters:
+        - errcheck
+      source: '(^\s*_\s*=|,\s*_)'
+    - path: 'tests/integration/e2e_conversion_test\.go'
+      linters:
+        - errcheck
+    - path: '(_test\.go|tests/)'
+      linters:
+        - revive
+      text: '^exported:'
+    - path: '(_test\.go|tests/)'
+      linters:
+        - gosec
+      text: 'G(101|401|501)'
+    - path: '(_test\.go|tests/)'
+      linters:
+        - gocyclo
+      text: 'cyclomatic complexity (1[1-9]|20) of .* is high \(> 10\)'
+    - linters:
+        - revive
+      text: '(that stutters|BuildUrl should be BuildURL|ConvertHttpRequest should be ConvertHTTPRequest|ConvertHttpResponse should be ConvertHTTPResponse)'
+    - path: 'internal/conversion/.*\.go'
+      linters:
+        - gocyclo
+        - gocritic
+    - path: '(internal/provider/client\.go|internal/service/model_service_impl\.go|internal/service/stats_buffer\.go|internal/handler/proxy_handler\.go|cmd/(desktop|server)/main\.go)'
+      linters:
+        - gocyclo