feat: add file serving endpoint GET /api/documents/{id}/file

Returns original document file from NAS. Fixes 404 on PDF/image
viewer in frontend. Updated frontend iframe/img src to match.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/routes/documents/[id]/+page.svelte

@@ -56,12 +56,12 @@
           </div>
         {:else if viewerType === 'pdf'}
           <iframe
-            src="/documents/file/{doc.id}"
+            src="/api/documents/{doc.id}/file"
             class="w-full h-[80vh] rounded"
             title={doc.title}
           ></iframe>
         {:else if viewerType === 'image'}
-          <img src="/documents/file/{doc.id}" alt={doc.title} class="max-w-full rounded" />
+          <img src="/api/documents/{doc.id}/file" alt={doc.title} class="max-w-full rounded" />
         {:else if viewerType === 'synology'}
           <div class="text-center py-10">
             <p class="text-[var(--text-dim)] mb-4">Synology Office 문서</p>