fix: store content type at upload time instead of guessing from extension

Previously FileService fell back to extension-based MIME detection, causing
TIFF, HEIC, DOCX and other unlisted types to be served as octet-stream
(forced download instead of inline display).

- Add content_type column to documents (V3 migration)
- Store file.getContentType() in DocumentService on upload and file replace
- MassImportService uses Files.probeContentType() for local files
- DocumentController prefers doc.getContentType() over S3-reported type
- FileService: remove extension-based fallback (no longer needed)
- DocumentService: replace leftover ResponseStatusException with DomainException

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

backend/src/main/resources/db/migration/V3__add_content_type_to_documents.sql

@@ -0,0 +1 @@
+ALTER TABLE documents ADD COLUMN content_type character varying(255);