feat(documents): extend quick-upload with optional batch metadata part

- Add DocumentBatchMetadataDTO (titles, senderId, receiverIds, documentDate, location, tags, metadataComplete)
- Add BATCH_TOO_LARGE to ErrorCode
- Extend quickUpload to accept optional @RequestPart("metadata"); dispatches to storeDocumentWithBatchMetadata when present
- Cap batch at 50 files/request; reject 400 when titles.size > files.size
- Add DocumentService.storeDocumentWithBatchMetadata applying shared fields + index-based titles to both created and updated docs
- Raise max-request-size to 500MB (10-file chunk at max per-file size)
- Add structured SLF4J logging for every quickUpload call

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/application.yaml

@@ -23,7 +23,8 @@ spring:
   servlet:
     multipart:
       max-file-size: 50MB
-      max-request-size: 50MB
+      max-request-size: 500MB   # supports 10-file chunk at max per-file size; see #317
+      file-size-threshold: 2KB
 
   mail:
     host: ${MAIL_HOST:}