feat: add PDF annotation feature (#40)

Backend:
- Add ANNOTATE_ALL permission
- Add ANNOTATION_NOT_FOUND and ANNOTATION_OVERLAP error codes
- V10 migration: document_annotations table with page/rect/color/owner
- DocumentAnnotation entity, AnnotationRepository, CreateAnnotationDTO
- AnnotationService: overlap detection (rectangle intersection), ownership enforcement on delete
- AnnotationController: GET (authenticated), POST/DELETE (ANNOTATE_ALL)
- 15 new tests (AnnotationServiceTest, AnnotationControllerTest) — TDD red/green

Frontend:
- AnnotationLayer.svelte: pointer-event drawing, colored rect overlays, delete buttons
- PdfViewer.svelte: annotate toggle, color picker, loads/saves/deletes annotations via API
- Disabled annotate button with tooltip for users without ANNOTATE_ALL
- canAnnotate exposed from layout server, passed to PdfViewer
- errors.ts + de/en/es translations for new error codes
- 3 new unit tests for AnnotationLayer — TDD red/green

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

frontend/messages/es.json

@@ -1,5 +1,7 @@
 {
 	"$schema": "https://inlang.com/schema/inlang-message-format",
+	"error_annotation_not_found": "Anotación no encontrada.",
+	"error_annotation_overlap": "La anotación se superpone con una existente.",
 	"error_document_not_found": "Documento no encontrado.",
 	"error_document_no_file": "No hay ningún archivo asociado a este documento.",
 	"error_file_not_found": "El archivo no pudo encontrarse en el almacenamiento.",