Serialize walks and backfill, split the ETA into start and run

poc/public/media.css

@@ -745,3 +745,8 @@
     background: color-mix(in srgb, var(--color-neutral-400) 18%, transparent);
     color: var(--color-neutral-300);
 }
+
+.vk-bot-eta {
+    cursor: help;
+    text-decoration: underline dotted color-mix(in srgb, var(--color-neutral-400) 60%, transparent);
+}

poc/public/media.js

@@ -552,6 +552,15 @@
         const bot = backend.slopbot ?? { pending: 0, cached: 0, failed: 0, total: 0 };
         const puppet = backend.puppet ?? { pending: 0, cached: 0, failed: 0, total: 0 };
 
+        const etaLine = el(
+            "div",
+            "vk-bot-summary-line vk-bot-eta",
+            `ETA ${formatEta(totals.etaTotalMs ?? totals.etaMs)} at ${totals.perMinute ?? 0} rows/min ` +
+                `(settings estimate ${status.estimatedPerMinute ?? 0}/min)`
+        );
+        etaLine.title =
+            `first job starts now \u00b7 queue drains in ${formatEta(totals.etaRunMs ?? totals.etaMs)}`;
+
         backfillSummary.replaceChildren(
             el(
                 "div",
@@ -560,12 +569,7 @@
                     `${totals.failed ?? 0} failed \u00b7 ${totals.downloaded ?? 0} downloaded \u00b7 ` +
                     `${formatBytes(totals.bytes ?? 0)} stored`
             ),
-            el(
-                "div",
-                "vk-bot-summary-line",
-                `ETA ${formatEta(totals.etaMs)} at ${totals.perMinute ?? 0} rows/min ` +
-                    `(settings estimate ${status.estimatedPerMinute ?? 0}/min)`
-            ),
+            etaLine,
             el(
                 "div",
                 "vk-bot-summary-line vk-dim",
@@ -590,8 +594,20 @@
             const parts = [`${job.pending} pending`];
             if (job.cached) parts.push(`${job.cached} cached`);
             if (job.failed) parts.push(`${job.failed} failed`);
-            if (job.pending > 0) parts.push(`ETA ${formatEta(job.etaMs)}`);
             head.append(el("span", "vk-bot-job-meta", parts.join(" \u00b7 ")));
+
+            if (job.pending > 0) {
+                const eta = el(
+                    "span",
+                    "vk-bot-job-meta vk-bot-eta",
+                    `ETA ${formatEta(job.etaTotalMs ?? job.etaMs)}`
+                );
+                eta.title =
+                    `starts in ${formatEta(job.etaStartMs ?? 0)} \u00b7 ` +
+                    `runs for ${formatEta(job.etaRunMs ?? job.etaMs)}`;
+                head.append(eta);
+            }
+
             row.append(head);
 
             for (const member of job.members) {