Fix provenance-backfill multipart offsets, incomplete-fingerprint fallback, and add name-size audit trail

Multipart ZIP fingerprint reads now use per-part sizes instead of the
whole-archive total, so the tail download offset stays within the last
part's bounds on both the scanned side and the destination-copy side
(scannedFileId replaced with an ordered scannedParts list). A fingerprint
comparison is now only treated as a real mismatch when both sides have
complete CRCs and differ; incomplete comparisons (e.g. empty files) fall
back to name+size confidence instead of silently refusing to backfill.
Name+size-confidence backfills now also create an INFO
INTEGRITY_AUDIT systemNotification for later review.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

worker/src/worker.ts

@@ -1673,7 +1673,7 @@ async function processOneArchiveSet(
         sourceCaption: archiveSet.parts[0].caption ?? null,
         remoteUniqueId: archiveSet.parts[0].remoteUniqueId ?? null,
         creator: derivedCreator,
-        scannedFileId: archiveSet.parts[archiveSet.parts.length - 1].fileId,
+        scannedParts: archiveSet.parts.map((p) => ({ fileId: p.fileId, fileSize: p.fileSize })),
         previewData: null,
         previewMsgId: preview?.id ?? null,
       });