feat: Workshop activity heatmap (24h job volume) (#9)

- Add GET /api/stats/activity returning job counts bucketed by UTC hour
  for the past 24 hours (GROUP BY date_trunc query via drizzle-orm)
- Add 24-segment SVG-less heatmap bar to the Workshop HUD; color
  intensity scales from dim to bright based on relative volume;
  current-hour segment pulses via CSS animation
- Hover tooltip shows "3pm: 12 jobs submitted" for each segment
- On mobile (≤600px), heatmap collapses to icon that opens a
  full-screen overlay
- Auto-refreshes every 5 minutes via setInterval

Fixes #9

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

artifacts/api-server/src/routes/index.ts

@@ -17,11 +17,13 @@ import relayRouter from "./relay.js";
 import adminRelayRouter from "./admin-relay.js";
 import adminRelayQueueRouter from "./admin-relay-queue.js";
 import geminiRouter from "./gemini.js";
+import statsRouter from "./stats.js";
 
 const router: IRouter = Router();
 
 router.use(healthRouter);
 router.use(metricsRouter);
+router.use(statsRouter);
 router.use(jobsRouter);
 router.use(estimateRouter);
 router.use(bootstrapRouter);

artifacts/api-server/src/routes/stats.ts

@@ -0,0 +1,59 @@
+import { Router, type Request, type Response } from "express";
+import { db, jobs } from "@workspace/db";
+import { sql, gte } from "drizzle-orm";
+import { makeLogger } from "../lib/logger.js";
+
+const router = Router();
+const logger = makeLogger("stats");
+
+/**
+ * GET /api/stats/activity
+ *
+ * Returns job counts bucketed by hour for the past 24 hours.
+ * Each bucket represents a UTC hour (0–23).
+ * Hours with no activity are included as 0.
+ *
+ * Response shape:
+ *   { hours: number[24], generatedAt: string }
+ *   hours[0] = oldest hour (24h ago), hours[23] = current hour
+ */
+router.get("/api/stats/activity", async (_req: Request, res: Response) => {
+  try {
+    const now = new Date();
+    const windowStart = new Date(now.getTime() - 24 * 60 * 60 * 1000);
+
+    // Count completed jobs grouped by the hour they were created,
+    // within the last 24h window.
+    const rows = await db
+      .select({
+        hour: sql<number>`cast(extract(epoch from date_trunc('hour', created_at)) as bigint)`,
+        count: sql<number>`cast(count(*) as int)`,
+      })
+      .from(jobs)
+      .where(gte(jobs.createdAt, windowStart))
+      .groupBy(sql`date_trunc('hour', created_at)`);
+
+    // Build a map: epoch-hour → count
+    const byEpochHour = new Map<number, number>();
+    for (const row of rows) {
+      byEpochHour.set(Number(row.hour), Number(row.count));
+    }
+
+    // Build 24-slot array aligned to whole hours, oldest first.
+    // slot 0 = floor(now - 24h), slot 23 = floor(now)
+    const currentHourEpoch = Math.floor(now.getTime() / (3600 * 1000)) * 3600;
+    const hours: number[] = [];
+    for (let i = 23; i >= 0; i--) {
+      const slotEpoch = currentHourEpoch - i * 3600;
+      hours.push(byEpochHour.get(slotEpoch) ?? 0);
+    }
+
+    res.json({ hours, generatedAt: now.toISOString() });
+  } catch (err) {
+    const message = err instanceof Error ? err.message : "Failed to fetch activity stats";
+    logger.error("activity stats failed", { error: message });
+    res.status(500).json({ error: message });
+  }
+});
+
+export default router;