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>

prisma/schema.prisma

@@ -522,6 +522,7 @@ model PackageGroup {
   name            String
   mediaAlbumId    String?
   sourceChannelId String
+  groupingSource  GroupingSource   @default(MANUAL)
   previewData     Bytes?
   createdAt       DateTime         @default(now())
   updatedAt       DateTime         @updatedAt
@@ -802,3 +803,45 @@ model KickstarterPackage {
   @@id([kickstarterId, packageId])
   @@map("kickstarter_packages")
 }
+
+// ── Grouping & Notifications ──
+
+enum GroupingSource {
+  ALBUM
+  MANUAL
+  AUTO_TIME
+  AUTO_PATTERN
+  AUTO_REPLY
+  AUTO_ZIP
+  AUTO_CAPTION
+}
+
+enum NotificationType {
+  HASH_MISMATCH
+  MISSING_PART
+  UPLOAD_FAILED
+  DOWNLOAD_FAILED
+  GROUPING_CONFLICT
+  INTEGRITY_AUDIT
+}
+
+enum NotificationSeverity {
+  INFO
+  WARNING
+  ERROR
+}
+
+model SystemNotification {
+  id        String               @id @default(cuid())
+  type      NotificationType
+  severity  NotificationSeverity @default(INFO)
+  title     String
+  message   String
+  context   Json?
+  isRead    Boolean              @default(false)
+  createdAt DateTime             @default(now())
+
+  @@index([isRead, createdAt])
+  @@index([type])
+  @@map("system_notifications")
+}