feat(scheduler): 引入调度快照缓存与 outbox 回放

- 调度热路径优先读 Redis 快照，保留分组排序语义
- outbox 回放 + 全量重建纠偏，失败重试不推进水位
- 自动 Atlas 基线对齐并同步调度配置示例

backend/migrations/036_scheduler_outbox.sql

@@ -0,0 +1,10 @@
+CREATE TABLE IF NOT EXISTS scheduler_outbox (
+    id BIGSERIAL PRIMARY KEY,
+    event_type TEXT NOT NULL,
+    account_id BIGINT NULL,
+    group_id BIGINT NULL,
+    payload JSONB NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_scheduler_outbox_created_at ON scheduler_outbox (created_at);