feat(upload): warn on duplicate filename with link to existing document

- storeDocument now returns StoreResult(document, isNew) to distinguish
  new uploads from updates to existing documents
- QuickUploadResult gains an `updated` list alongside `created`
- Frontend shows an amber warning with a "View document" link for duplicates
  instead of silently re-uploading and leaving the user confused

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

frontend/src/routes/+page.svelte

@@ -25,7 +25,7 @@ let isDragging = $state(false);
 let windowDragging = $state(false);
 let dragCounter = 0;
 let isUploading = $state(false);
-let uploadMessages = $state<{ text: string; isError: boolean }[]>([]);
+let uploadMessages = $state<{ text: string; isError: boolean; link?: string }[]>([]);
 let fileInput: HTMLInputElement;
 
 let searchTimer: ReturnType<typeof setTimeout>;
@@ -67,7 +67,7 @@ async function handleFileSelect(e: Event) {
 async function uploadFiles(files: File[]) {
 	if (files.length === 0) return;
 
-	const messages: { text: string; isError: boolean }[] = [];
+	const messages: { text: string; isError: boolean; link?: string }[] = [];
 
 	// Client-side type validation
 	const valid: File[] = [];
@@ -101,6 +101,13 @@ async function uploadFiles(files: File[]) {
 			if (result.created?.length > 0) {
 				messages.push({ text: m.upload_success({ count: result.created.length }), isError: false });
 			}
+			for (const doc of result.updated ?? []) {
+				messages.push({
+					text: m.upload_duplicate({ filename: doc.originalFilename }),
+					isError: false,
+					link: `/documents/${doc.id}`
+				});
+			}
 			for (const err of result.errors ?? []) {
 				messages.push({
 					text: `${err.filename}: ${getErrorMessage(err.code)}`,
@@ -374,8 +381,18 @@ $effect(() => {
 		{#if uploadMessages.length > 0}
 			<div class="mb-4 flex flex-col gap-1">
 				{#each uploadMessages as msg, i (i)}
-					<p class="font-sans text-sm {msg.isError ? 'text-red-600' : 'text-green-700'}">
+					<p
+						class="font-sans text-sm {msg.isError
+							? 'text-red-600'
+							: msg.link
+								? 'text-amber-700'
+								: 'text-green-700'}"
+					>
 						{msg.text}
+						{#if msg.link}
+							<a href={msg.link} class="underline hover:no-underline">{m.upload_duplicate_link()}</a
+							>
+						{/if}
 					</p>
 				{/each}
 			</div>