feat: grouping phase 1 — schema, ungrouped tab, time-window grouping, hash verification

Schema:
- Add GroupingSource enum (ALBUM, MANUAL, AUTO_TIME, AUTO_PATTERN, etc.)
- Add groupingSource field to PackageGroup with backfill
- Add SystemNotification model for persistent alerts
- Add NotificationType and NotificationSeverity enums

Ungrouped staging tab:
- Add listUngroupedPackages/countUngroupedPackages queries
- Add "Ungrouped" tab to STL page showing packages without a group

Time-window auto-grouping:
- After album grouping, cluster ungrouped packages within configurable
  time window (default 5 min, AUTO_GROUP_TIME_WINDOW_MINUTES env var)
- Groups named from common filename prefix
- Groups created with groupingSource=AUTO_TIME

Hash verification after split:
- Re-hash split parts and compare to original contentHash
- Log error and create SystemNotification on mismatch
- Prevents silently corrupted split uploads

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

worker/src/db/queries.ts

@@ -587,3 +587,24 @@ export async function linkPackagesToGroup(
     data: { packageGroupId: groupId },
   });
 }
+
+export async function createTimeWindowGroup(input: {
+  sourceChannelId: string;
+  name: string;
+  packageIds: string[];
+}): Promise<string> {
+  const group = await db.packageGroup.create({
+    data: {
+      sourceChannelId: input.sourceChannelId,
+      name: input.name,
+      groupingSource: "AUTO_TIME",
+    },
+  });
+
+  await db.package.updateMany({
+    where: { id: { in: input.packageIds } },
+    data: { packageGroupId: group.id },
+  });
+
+  return group.id;
+}