feat(订阅配置): 支持环境变量展开并校验订阅连接地址

Motivation:
订阅连接地址中的敏感凭据不应硬编码到配置文件，改为通过环境变量注入，并自动跳过未配置或非 AMQP 协议的订阅，避免无效连接导致启动失败。

Changes:

* 订阅 URL 加载时展开环境变量并去除首尾空白
* 新增 AMQP 协议校验，仅 amqp/amqps 地址视为有效连接
* 非 AMQP 协议的订阅（如未配置的占位符）不再参与激活
* docker-compose 环境变量改用映射格式并加引号，规避特殊字符解析问题
* 补充环境变量展开与占位符跳过相关的单元测试

internal/config/config_test.go

@@ -1,6 +1,10 @@
 package config
 
-import "testing"
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
 
 func TestNormalizeSubscriptionDefaults(t *testing.T) {
 	cfg := &Config{Subscriptions: []SubscriptionConfig{{
@@ -47,6 +51,47 @@ func TestNormalizeUnknownFormatter(t *testing.T) {
 	}
 }
 
+func TestLoadExpandsRabbitMQURL(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.yaml")
+	yaml := []byte(`
+server:
+  port: 8080
+  admin_key: test
+subscriptions:
+  - name: trade-signal
+    url: "${RABBITMQ_URL}"
+    queue: q
+    source: trade-signal
+`)
+	if err := os.WriteFile(path, yaml, 0o644); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("RABBITMQ_URL", "amqps://user:pass@example.invalid:5671/vhost")
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(cfg.Subscriptions) != 1 {
+		t.Fatalf("subs=%d", len(cfg.Subscriptions))
+	}
+	if cfg.Subscriptions[0].URL != "amqps://user:pass@example.invalid:5671/vhost" {
+		t.Fatalf("url=%q", cfg.Subscriptions[0].URL)
+	}
+	if n := len(cfg.ActiveSubscriptions()); n != 1 {
+		t.Fatalf("active=%d", n)
+	}
+}
+
+func TestActiveSubscriptionsSkipsPlaceholderURL(t *testing.T) {
+	cfg := &Config{Subscriptions: []SubscriptionConfig{{
+		URL: "${RABBITMQ_URL}", Queue: "q", Source: "s", Name: "trade-signal",
+	}}}
+	if n := len(cfg.ActiveSubscriptions()); n != 0 {
+		t.Fatalf("placeholder should not be active, n=%d", n)
+	}
+}
+
 func TestQuantityMultiplierFor(t *testing.T) {
 	o := StrategyOverride{QuantityMultipliers: QuantityMultipliers{Open: 100, Add: 0}}
 	if o.QuantityMultiplierFor("OPEN") != 100 {