feat: pattern/creator grouping, notification UI, failure alerts

Pattern grouping (Signal 3):
- Extract YYYY-MM dates, month names, and project prefixes from filenames
- Auto-group packages sharing the same pattern within a channel
- Groups created with groupingSource=AUTO_PATTERN

Creator grouping (Signal 4):
- Auto-group 3+ ungrouped packages from the same creator within a channel
- Runs after pattern grouping as lowest-priority automatic signal

Notification UI:
- Add NotificationBell component to header with unread badge
- Popover panel shows recent notifications with severity icons
- Mark individual or all notifications as read
- Polls every 30 seconds for updates

Failure notifications:
- Upload/download failures now create SystemNotification records
- Visible in the notification bell alongside hash mismatch alerts

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/app/api/notifications/route.ts

@@ -0,0 +1,27 @@
+import { NextResponse } from "next/server";
+import { auth } from "@/lib/auth";
+import {
+  getRecentNotifications,
+  getUnreadNotificationCount,
+} from "@/data/notification.queries";
+
+export const dynamic = "force-dynamic";
+
+export async function GET() {
+  const session = await auth();
+  if (!session?.user?.id) {
+    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  }
+
+  const [notifications, unreadCount] = await Promise.all([
+    getRecentNotifications(30),
+    getUnreadNotificationCount(),
+  ]);
+
+  const serialized = notifications.map((n) => ({
+    ...n,
+    createdAt: n.createdAt.toISOString(),
+  }));
+
+  return NextResponse.json({ notifications: serialized, unreadCount });
+}