feat(backend): hash uploaded files and store hash on documents and annotations

- Flyway V13: add file_hash column to documents and document_annotations
- FileService.uploadFile() now returns UploadResult(s3Key, fileHash) with SHA-256 hash computed from raw bytes
- Document and DocumentAnnotation models gain a fileHash field
- DocumentService propagates the hash at all three upload sites (storeDocument, createDocument, updateDocument)
- AnnotationService.createAnnotation() accepts and persists a fileHash
- AnnotationController resolves the document's hash and passes it through

Closes #55

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

backend/src/main/resources/db/migration/V13__add_file_hash.sql

@@ -0,0 +1,7 @@
+-- Add content-based file hash to documents for annotation versioning
+ALTER TABLE documents
+    ADD COLUMN file_hash VARCHAR(64);
+
+-- Each annotation remembers which file version it was created against
+ALTER TABLE document_annotations
+    ADD COLUMN file_hash VARCHAR(64);