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/util/config.ts

@@ -10,6 +10,8 @@ export const config = {
   /** Maximum file part size for Telegram upload (in MiB). Default 1950 (under 2GB non-Premium limit).
    *  Set to 3900 for Premium accounts (under 4GB limit). */
   maxPartSizeMB: parseInt(process.env.MAX_PART_SIZE_MB ?? "1950", 10),
+  /** Time window for auto-grouping ungrouped packages from the same channel (minutes). 0 = disabled. */
+  autoGroupTimeWindowMinutes: parseInt(process.env.AUTO_GROUP_TIME_WINDOW_MINUTES ?? "5", 10),
   /** Maximum jitter added to scheduler interval (in minutes) */
   jitterMinutes: 5,
   /** Maximum time span for multipart archive parts (in hours). 0 = no limit. */