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/src/routes/+layout.server.ts

@@ -1,11 +1,10 @@
 import type { LayoutServerLoad } from './$types';
 
 export const load: LayoutServerLoad = async ({ locals }) => {
+	const groups: { permissions: string[] }[] = locals.user?.groups ?? [];
 	return {
 		user: locals.user,
-		canWrite:
-			locals.user?.groups?.some((g: { permissions: string[] }) =>
-				g.permissions.includes('WRITE_ALL')
-			) ?? false
+		canWrite: groups.some((g) => g.permissions.includes('WRITE_ALL')),
+		canAnnotate: groups.some((g) => g.permissions.includes('ANNOTATE_ALL'))
 	};
 };