feat(stats): 新增聚合读路径与回填机制并重构 dashboard/usage 读取链路

- 新增 stats_user_summary 及 user_daily_provider/api_format/cost_savings 等聚合表
- 扩展 stats_daily/hourly 有效 token 与响应时间等字段，maintenance runtime 同步写入
- 新增 backfill 模块与 --apply-backfills 命令补齐历史聚合数据
- 重写 dashboard_filters、usage_heatmap、user_rollups 查询改走聚合表
- 同步更新 baseline_v2.sql 与 migration 集，README/dev.sh 补充回填用法

apps/aether-gateway/src/state/core.rs

@@ -255,6 +255,14 @@ impl AppState {
         Ok(true)
     }
 
+    pub async fn run_postgres_backfills(&self) -> Result<bool, sqlx::migrate::MigrateError> {
+        let Some(pool) = self.postgres_pool() else {
+            return Ok(false);
+        };
+        aether_data::backfill::run_backfills(&pool).await?;
+        Ok(true)
+    }
+
     pub async fn pending_postgres_migrations(
         &self,
     ) -> Result<Option<Vec<aether_data::migrate::PendingMigrationInfo>>, sqlx::migrate::MigrateError>
@@ -277,6 +285,16 @@ impl AppState {
         ))
     }
 
+    pub async fn pending_postgres_backfills(
+        &self,
+    ) -> Result<Option<Vec<aether_data::backfill::PendingBackfillInfo>>, sqlx::migrate::MigrateError>
+    {
+        let Some(pool) = self.postgres_pool() else {
+            return Ok(None);
+        };
+        Ok(Some(aether_data::backfill::pending_backfills(&pool).await?))
+    }
+
     pub fn with_video_task_poller_config(mut self, interval: Duration, batch_size: usize) -> Self {
         self.video_task_poller = Some(VideoTaskPollerConfig {
             interval,