feat(enrich): add metadata enrichment queue UI

Home page shows "Needs metadata" card when incomplete documents exist.
/enrich list shows all incomplete documents; /enrich/[id] provides a
split PDF-preview + compact form view with Skip / Save / Save & reviewed
actions that auto-advance through the queue.

New document page gets Save vs Save & reviewed split. Edit page gets
"Mark for review" secondary button to push a document back into the queue.

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

frontend/messages/en.json

@@ -274,5 +274,21 @@
 	"upload_duplicate": "{filename} already exists —",
 	"upload_duplicate_link": "View document",
 	"upload_invalid_type": "{filename}: unsupported file format",
-	"upload_error": "Error uploading {filename}"
+	"upload_error": "Error uploading {filename}",
+	"enrich_list_back": "Back to overview",
+	"enrich_list_count": "documents",
+	"btn_save_and_mark_reviewed": "Save & mark as reviewed",
+	"btn_mark_for_review": "Mark for review",
+	"enrich_needs_metadata_title": "Documents without metadata",
+	"enrich_needs_metadata_count": "{count} document(s) waiting for metadata",
+	"enrich_needs_metadata_cta": "Complete now",
+	"enrich_list_heading": "Documents without metadata",
+	"enrich_list_empty_heading": "All documents complete",
+	"enrich_list_empty_body": "There are no documents that still need metadata.",
+	"enrich_list_start": "Start reviewing",
+	"enrich_progress": "{count} remaining",
+	"enrich_skip": "Skip",
+	"enrich_done_heading": "All done!",
+	"enrich_done_body": "All documents have been processed.",
+	"enrich_back_to_list": "Back to list"
 }