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/migrations/0101_worker_adaptive_load.sql

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+ALTER TABLE gateway_worker_instances
+  ADD COLUMN hard_capacity_limit integer NOT NULL DEFAULT 0,
+  ADD COLUMN safe_capacity integer NOT NULL DEFAULT 0,
+  ADD COLUMN heavy_capacity integer NOT NULL DEFAULT 0,
+  ADD COLUMN active_tasks integer NOT NULL DEFAULT 0,
+  ADD COLUMN preparing_tasks integer NOT NULL DEFAULT 0,
+  ADD COLUMN waiting_upstream_tasks integer NOT NULL DEFAULT 0,
+  ADD COLUMN finalizing_tasks integer NOT NULL DEFAULT 0,
+  ADD COLUMN pressure_state text NOT NULL DEFAULT 'unknown',
+  ADD COLUMN pressure_reason text NOT NULL DEFAULT '',
+  ADD COLUMN load_sampled_at timestamptz,
+  ADD CONSTRAINT gateway_worker_instances_adaptive_capacity_check
+    CHECK (
+      hard_capacity_limit >= 0
+      AND safe_capacity >= 0
+      AND heavy_capacity >= 0
+      AND active_tasks >= 0
+      AND preparing_tasks >= 0
+      AND waiting_upstream_tasks >= 0
+      AND finalizing_tasks >= 0
+      AND active_tasks = preparing_tasks + waiting_upstream_tasks + finalizing_tasks
+    ) NOT VALID,
+  ADD CONSTRAINT gateway_worker_instances_pressure_state_check
+    CHECK (pressure_state IN ('unknown', 'normal', 'busy', 'critical')) NOT VALID;
+
+ALTER TABLE gateway_concurrency_leases
+  ADD COLUMN limit_value numeric,
+  ADD CONSTRAINT gateway_concurrency_leases_limit_value_check
+    CHECK (limit_value IS NULL OR limit_value > 0) NOT VALID;
+
+UPDATE gateway_worker_instances
+SET hard_capacity_limit = capacity_limit,
+    safe_capacity = capacity_limit,
+    heavy_capacity = capacity_limit
+WHERE hard_capacity_limit = 0;
+
+ALTER TABLE gateway_worker_instances
+  VALIDATE CONSTRAINT gateway_worker_instances_adaptive_capacity_check;
+
+ALTER TABLE gateway_worker_instances
+  VALIDATE CONSTRAINT gateway_worker_instances_pressure_state_check;
+
+ALTER TABLE gateway_concurrency_leases
+  VALIDATE CONSTRAINT gateway_concurrency_leases_limit_value_check;