feat(config): add ${ENV_VAR} interpolation in YAML configuration

Enable environment variable substitution in configuration files using
${VAR} syntax. Supports 12-factor app deployment patterns without
hardcoding secrets or environment-specific values.

Syntax:
- Only ${VAR} with curly braces (avoids conflict with $variable system)
- Missing variables preserved as-is (${MISSING} stays unchanged)
- Multiple variables per line supported
- Adjacent variables ${A}${B} handled correctly

Integration:
- Applied in config.Load() after os.ReadFile, before yaml.Unmarshal
- Applied in processIncludes() for each included file
- 12 unit tests covering single/multiple/missing/empty variables

internal/config/config.go

@@ -134,6 +134,8 @@ func Load(path string) (*Config, error) {
 		return nil, fmt.Errorf("读取配置文件失败: %w", err)
 	}
 
+	data = ExpandEnv(data)
+
 	// 从默认值开始，YAML 只覆盖显式配置的字段。
 	// 注意：yaml.v3 对 slice 会整体替换，因此用户显式配置的 Servers[]
 	// 元素不会继承 server-level 默认值；但顶层 struct 字段（Performance、
@@ -206,6 +208,8 @@ func processIncludes(cfg *Config, baseDir string, depth int, visited map[string]
 				return fmt.Errorf("读取引入文件 %q 失败: %w", match, err)
 			}
 
+			data = ExpandEnv(data)
+
 			var included Config
 			if err := yaml.Unmarshal(data, &included); err != nil {
 				return fmt.Errorf("解析引入文件 %q 失败: %w", match, err)

internal/config/env.go

@@ -0,0 +1,22 @@
+package config
+
+import (
+	"os"
+	"regexp"
+)
+
+var envPattern = regexp.MustCompile(`\$\{([^}]+)\}`)
+
+// ExpandEnv replaces ${VAR} patterns with environment variable values.
+func ExpandEnv(data []byte) []byte {
+	return envPattern.ReplaceAllFunc(data, func(match []byte) []byte {
+		name := string(match[2 : len(match)-1])
+		if name == "" {
+			return match
+		}
+		if value, ok := os.LookupEnv(name); ok {
+			return []byte(value)
+		}
+		return match
+	})
+}

internal/config/env_test.go

@@ -0,0 +1,121 @@
+package config
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestExpandEnv(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected string
+		setup    func(*testing.T)
+	}{
+		{
+			name:     "single variable",
+			input:    "${VAR}",
+			expected: "value",
+			setup: func(t *testing.T) {
+				t.Setenv("VAR", "value")
+			},
+		},
+		{
+			name:     "multiple variables",
+			input:    "${HOST}:${PORT}",
+			expected: "localhost:8080",
+			setup: func(t *testing.T) {
+				t.Setenv("HOST", "localhost")
+				t.Setenv("PORT", "8080")
+			},
+		},
+		{
+			name:     "variable with prefix and suffix",
+			input:    "prefix_${VAR}_suffix",
+			expected: "prefix_value_suffix",
+			setup: func(t *testing.T) {
+				t.Setenv("VAR", "value")
+			},
+		},
+		{
+			name:     "missing variable unchanged",
+			input:    "${MISSING}",
+			expected: "${MISSING}",
+		},
+		{
+			name:     "mixed existing and missing",
+			input:    "${VAR}/${MISSING}",
+			expected: "value/${MISSING}",
+			setup: func(t *testing.T) {
+				t.Setenv("VAR", "value")
+			},
+		},
+		{
+			name:     "empty input",
+			input:    "",
+			expected: "",
+		},
+		{
+			name:     "no variables",
+			input:    "just a plain string",
+			expected: "just a plain string",
+		},
+		{
+			name:     "empty variable name",
+			input:    "${}",
+			expected: "${}",
+		},
+		{
+			name:     "adjacent variables",
+			input:    "${VAR1}${VAR2}",
+			expected: "value1value2",
+			setup: func(t *testing.T) {
+				t.Setenv("VAR1", "value1")
+				t.Setenv("VAR2", "value2")
+			},
+		},
+		{
+			name:     "variable with empty value",
+			input:    "${EMPTY_VAR}",
+			expected: "",
+			setup: func(t *testing.T) {
+				t.Setenv("EMPTY_VAR", "")
+			},
+		},
+		{
+			name:     "same variable multiple times",
+			input:    "${VAR}-${VAR}-${VAR}",
+			expected: "val-val-val",
+			setup: func(t *testing.T) {
+				t.Setenv("VAR", "val")
+			},
+		},
+		{
+			name: "full yaml-like input",
+			input: `server:
+  host: ${HOST}
+  port: ${PORT}
+  name: ${APP_NAME}`,
+			expected: `server:
+  host: 127.0.0.1
+  port: 9090
+  name: lolly`,
+			setup: func(t *testing.T) {
+				t.Setenv("HOST", "127.0.0.1")
+				t.Setenv("PORT", "9090")
+				t.Setenv("APP_NAME", "lolly")
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if tt.setup != nil {
+				tt.setup(t)
+			}
+			result := ExpandEnv([]byte(tt.input))
+			assert.Equal(t, tt.expected, string(result))
+		})
+	}
+}