fix: improve sql queries

apps/shared/db.ts

@@ -243,6 +243,17 @@ export async function migrate(sql: any) {
   `;
   await sql`CREATE INDEX IF NOT EXISTS idx_uptime_rollup_lookup ON monitor_uptime_rollup(monitor_id, bucket_type, bucket_start DESC)`;
 
+  // Watermark table for the rollup job. Each row records the most recent
+  // pings.checked_at that has been folded into a given bucket_type's rollup
+  // rows. Lets the periodic rollup pass scan only NEW pings since the last
+  // pass instead of re-aggregating the entire current bucket on every tick.
+  await sql`
+    CREATE TABLE IF NOT EXISTS rollup_watermarks (
+      bucket_type           TEXT PRIMARY KEY,
+      last_aggregated_at    TIMESTAMPTZ NOT NULL DEFAULT to_timestamp(0)
+    )
+  `;
+
   await sql`CREATE INDEX IF NOT EXISTS idx_pings_monitor ON pings(monitor_id, checked_at DESC)`;
   await sql`CREATE INDEX IF NOT EXISTS idx_pings_checked_at ON pings(checked_at)`;