feat: upload PDF file on save via multipart FormData

Switch save endpoint from JSON to multipart upload so the PDF file is
sent alongside extracted data and tags.

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

frontend/src/pages/ExtractPage.vue

@@ -250,17 +250,17 @@ async function handleTaggingSave(depensesTags, shouldOverwrite) {
   saveMessage.value = null
   
   try {
-    const response = await fetch('/api/save', {
+    const formData = new FormData()
+    formData.append('pdf_file', pdfFile.value)
+    formData.append('data', JSON.stringify(extractedData.value.data))
+    if (depensesTags && depensesTags.length > 0) {
+      formData.append('depenses_tags', JSON.stringify(depensesTags))
+    }
+    formData.append('overwrite', shouldOverwrite ? 'true' : 'false')
+
+    const response = await fetch('/api/save-with-pdf', {
       method: 'POST',
-      headers: {
-        'Content-Type': 'application/json'
-      },
-      body: JSON.stringify({
-        source_file: extractedData.value.source_file,
-        data: extractedData.value.data,
-        depenses_tags: depensesTags,
-        overwrite: shouldOverwrite || false
-      })
+      body: formData
     })
     
     const result = await response.json()