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/components/layout/header.tsx

@@ -6,6 +6,7 @@ import { Button } from "@/components/ui/button";
 import { Sheet, SheetContent, SheetTrigger } from "@/components/ui/sheet";
 import { UserMenu } from "./user-menu";
 import { MobileSidebar } from "./mobile-sidebar";
+import { NotificationBell } from "./notification-bell";
 
 const routeTitles: Record<string, string> = {
   "/dashboard": "Dashboard",
@@ -38,7 +39,8 @@ export function Header() {
 
       <h1 className="text-lg font-semibold">{title}</h1>
 
-      <div className="ml-auto">
+      <div className="ml-auto flex items-center gap-1">
+        <NotificationBell />
         <UserMenu />
       </div>
     </header>