fix(upload): structured error codes for quick-upload, fix duplicate filename crash

- Switch errors from plain strings to { filename, code } objects so the
  frontend can show translated messages instead of raw exception text
- Add UNSUPPORTED_FILE_TYPE error code end-to-end (Java enum → errors.ts
  → de/en/es messages)
- Fix IncorrectResultSizeDataAccessException when a filename exists more
  than once in the DB: use findFirstByOriginalFilename instead of
  findByOriginalFilename in storeDocument()

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

frontend/src/routes/+page.svelte

@@ -7,6 +7,7 @@ import { untrack } from 'svelte';
 import { SvelteURLSearchParams } from 'svelte/reactivity';
 import { m } from '$lib/paraglide/messages.js';
 import { formatDate } from '$lib/utils/date';
+import { getErrorMessage } from '$lib/errors';
 
 let { data } = $props();
 
@@ -101,7 +102,10 @@ async function uploadFiles(files: File[]) {
 				messages.push({ text: m.upload_success({ count: result.created.length }), isError: false });
 			}
 			for (const err of result.errors ?? []) {
-				messages.push({ text: err, isError: true });
+				messages.push({
+					text: `${err.filename}: ${getErrorMessage(err.code)}`,
+					isError: true
+				});
 			}
 			await invalidateAll();
 		} else {