feat(worker): 实现集群限流与自适应负载

保留平台模型 RPM、TPM 和并发策略语义，增加 PostgreSQL 集群级租约、饱和候选重选和多平台自动负载，避免突发任务固定等待首个平台。\n\n新增 Worker 实时负载采样、自适应 active/heavy 容量、心跳与管理端指标，并扩展本地 acceptance runner，覆盖三 Worker、同模型三平台 2/4/6 并发和 48 个带图视频突发任务。\n\n验证：go test ./...、go vet ./...、PostgreSQL 跨 Store 集成测试、gofmt、bash -n、ShellCheck 及本地集群 provider-burst 验收通过；48/48 成功，无越限、重复提交、重复计费、重复回调或终态资源泄漏。

apps/api/internal/acceptanceworkload/profiles.go

@@ -5,6 +5,7 @@ import "time"
 type GeminiProfile struct {
 	Name        string
 	Requests    int
+	InputImages int
 	InputBytes  int
 	OutputBytes int
 	DelayMin    time.Duration
@@ -13,15 +14,19 @@ type GeminiProfile struct {
 
 var (
 	GeminiBaseline = GeminiProfile{
-		Name: "gemini-baseline", Requests: 1000, InputBytes: 256 << 10, OutputBytes: 256 << 10,
+		Name: "gemini-baseline", Requests: 1000, InputImages: 1, InputBytes: 256 << 10, OutputBytes: 256 << 10,
 		DelayMin: 4 * time.Second, DelayMax: 4 * time.Second,
 	}
+	GeminiMultiImage = GeminiProfile{
+		Name: "gemini-multi-image", Requests: 192, InputImages: 3, InputBytes: 768 << 10, OutputBytes: 0,
+		DelayMin: 8 * time.Second, DelayMax: 15 * time.Second,
+	}
 	GeminiLarge = GeminiProfile{
-		Name: "gemini-large", Requests: 128, InputBytes: 2 << 20, OutputBytes: 4 << 20,
+		Name: "gemini-large", Requests: 128, InputImages: 1, InputBytes: 2 << 20, OutputBytes: 4 << 20,
 		DelayMin: 8 * time.Second, DelayMax: 15 * time.Second,
 	}
 	GeminiPeak = GeminiProfile{
-		Name: "gemini-peak", Requests: 32, InputBytes: 8 << 20, OutputBytes: 8 << 20,
+		Name: "gemini-peak", Requests: 32, InputImages: 1, InputBytes: 8 << 20, OutputBytes: 8 << 20,
 		DelayMin: 15 * time.Second, DelayMax: 30 * time.Second,
 	}
 )
@@ -30,6 +35,8 @@ func GeminiProfileByName(name string) (GeminiProfile, bool) {
 	switch name {
 	case GeminiBaseline.Name:
 		return GeminiBaseline, true
+	case GeminiMultiImage.Name:
+		return GeminiMultiImage, true
 	case GeminiLarge.Name:
 		return GeminiLarge, true
 	case GeminiPeak.Name: