fix(交易信号): 修复极小价格展示精度丢失与策略覆盖匹配失效

Motivation:
部分 meme 币等标的价格极小（如 0.00000059），原有格式化统一保留两位小数会将其显示为 0.00，导致推送消息中的价格信息失真、误导用户；同时 viper 加载配置时会将嵌套 map 的 key 统一转为小写，导致按大写策略编码配置的策略覆盖项无法命中，仓位倍数、杠杆等覆盖参数失效。

Changes:

* 新增价格展示格式化逻辑：绝对值不小于 1 的数值保留两位小数，小于 1 的数值采用最短精确表示，避免极小价格被截断为 0.00
* 策略覆盖查找改为大小写不敏感匹配，兼容 viper 将配置 key 小写化的行为，确保策略编码以任意大小写配置均可生效
* 补充极小价格展示、配置加载解析及策略覆盖匹配的单元测试

internal/config/config_test.go

@@ -92,6 +92,50 @@ func TestActiveSubscriptionsSkipsPlaceholderURL(t *testing.T) {
 	}
 }
 
+func TestLoadParsesStrategyOverrides(t *testing.T) {
+	cfg, err := Load(filepath.Join("..", "..", "config", "config.yaml"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	var sub *SubscriptionConfig
+	for i := range cfg.Subscriptions {
+		if cfg.Subscriptions[i].Name == "trade-signal" {
+			sub = &cfg.Subscriptions[i]
+			break
+		}
+	}
+	if sub == nil {
+		t.Fatal("trade-signal subscription not found")
+	}
+	if len(sub.StrategyOverrides) == 0 {
+		t.Fatalf("strategy_overrides not parsed: %+v", *sub)
+	}
+	// Viper lower-cases nested map keys during load, so the parsed key is "blong".
+	o, ok := sub.StrategyOverrides["blong"]
+	if !ok {
+		t.Fatalf("blong override missing, got keys=%v", sub.StrategyOverrides)
+	}
+	got := o.QuantityMultiplierFor("OPEN")
+	if got != 100 {
+		t.Fatalf("open multiplier=%v want 100", got)
+	}
+	if o.Leverage == nil || *o.Leverage != 100 {
+		t.Fatalf("leverage=%v want 100", o.Leverage)
+	}
+}
+
+func TestLoadStrategyOverridesKeyInsensitive(t *testing.T) {
+	// Confirms viper lower-cases keys; tradesignal.overrideFor matches case-insensitively.
+	sub := SubscriptionConfig{StrategyOverrides: map[string]StrategyOverride{
+		"blong": {Leverage: intPtr(100)},
+	}}
+	if _, ok := sub.StrategyOverrides["BLONG"]; ok {
+		t.Fatalf("expected case-sensitive map; overrides=%v", sub.StrategyOverrides)
+	}
+}
+
+func intPtr(v int) *int { return &v }
+
 func TestQuantityMultiplierFor(t *testing.T) {
 	o := StrategyOverride{QuantityMultipliers: QuantityMultipliers{Open: 100, Add: 0}}
 	if o.QuantityMultiplierFor("OPEN") != 100 {