修复(#4): 抽奖发奖修正 — 保底奖不参与随机 + 无订阅时按套餐自动新建订阅

- weighted_picker: Pick 排除 is_fallback 保底奖，避免真实奖被"谢谢参与"挤占
- vpn_duration handler: 无活跃订阅且奖品配置 subscribe_id 时，按该套餐在抽奖
  事务内新建订阅并发放时长；未配置则沿用旧的安全跳过
- 补充单测：picker 排除保底奖、vpn_duration 自动新建订阅、draw 端到端链路

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/model/lottery/weighted_picker_test.go

@@ -58,6 +58,34 @@ func TestWeightedPicker_DistributionCloseToWeights(t *testing.T) {
 	}
 }
 
+func TestWeightedPicker_ExcludesFallback(t *testing.T) {
+	p := NewWeightedPicker(3)
+	pool := []Prize{
+		{Id: 1, Weight: 100, IsFallback: true},         // 保底奖：即便权重很高也绝不被随机抽中
+		{Id: 2, Weight: 5, Type: PrizeTypeVPNDuration}, // 唯一可抽真实奖品
+	}
+	for i := 0; i < 300; i++ {
+		idx, err := p.Pick(pool)
+		if err != nil {
+			t.Fatalf("Pick err: %v", err)
+		}
+		if idx != 1 {
+			t.Fatalf("fallback prize must never be picked; expected idx 1, got %d", idx)
+		}
+	}
+}
+
+func TestWeightedPicker_AllFallbackYieldsEmptyPool(t *testing.T) {
+	p := NewWeightedPicker(9)
+	pool := []Prize{
+		{Id: 1, Weight: 10, IsFallback: true},
+		{Id: 2, Weight: 20, IsFallback: true},
+	}
+	if _, err := p.Pick(pool); err != ErrEmptyPool {
+		t.Fatalf("expected ErrEmptyPool when only fallback prizes exist, got %v", err)
+	}
+}
+
 func TestWeightedPicker_DeterministicWithFixedSeed(t *testing.T) {
 	pool := []Prize{
 		{Id: 1, Weight: 1},