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/internal/conversion/openai/supplemental_test.go

@@ -177,7 +177,8 @@ func TestEncodeRerankResponse(t *testing.T) {
 	var result map[string]any
 	require.NoError(t, json.Unmarshal(body, &result))
 	assert.Equal(t, "rerank-1", result["model"])
-	results := result["results"].([]any)
+	results, okr := result["results"].([]any)
+	require.True(t, okr)
 	assert.Len(t, results, 1)
 }
 
@@ -356,9 +357,9 @@ func TestEncodeResponse_UsageWithCacheAndReasoning(t *testing.T) {
 	reasoning := 20
 	sr := canonical.StopReasonEndTurn
 	resp := &canonical.CanonicalResponse{
-		ID:      "r1",
-		Model:   "gpt-4",
-		Content: []canonical.ContentBlock{canonical.NewTextBlock("ok")},
+		ID:         "r1",
+		Model:      "gpt-4",
+		Content:    []canonical.ContentBlock{canonical.NewTextBlock("ok")},
 		StopReason: &sr,
 		Usage: canonical.CanonicalUsage{
 			InputTokens:     100,
@@ -373,7 +374,8 @@ func TestEncodeResponse_UsageWithCacheAndReasoning(t *testing.T) {
 
 	var result map[string]any
 	require.NoError(t, json.Unmarshal(body, &result))
-	usage := result["usage"].(map[string]any)
+	usage, oku := result["usage"].(map[string]any)
+	require.True(t, oku)
 	assert.Equal(t, float64(100), usage["prompt_tokens"])
 	ptd, ok := usage["prompt_tokens_details"].(map[string]any)
 	require.True(t, ok)
@@ -412,8 +414,10 @@ func TestEncodeResponse_StopReasons(t *testing.T) {
 
 			var result map[string]any
 			require.NoError(t, json.Unmarshal(body, &result))
-			choices := result["choices"].([]any)
-			choice := choices[0].(map[string]any)
+			choices, okch := result["choices"].([]any)
+			require.True(t, okch)
+			choice, okc := choices[0].(map[string]any)
+			require.True(t, okc)
 			assert.Equal(t, tt.want, choice["finish_reason"])
 		})
 	}