Add timestamp to CSV/SQL export filenames

linkvault-links.csv/.sql -> linkvault-links_2026-07-30_1542.csv/.sql,
so repeated exports don't just overwrite the same downloaded file.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/main.py

@@ -835,10 +835,13 @@ def export_csv(request: Request, db: Session = Depends(get_db)):
             select(Link).where(Link.user_id == user.id).order_by(Link.created_at)
         ).all()
     )
+    stamp = datetime.now().strftime("%Y-%m-%d_%H%M")
     return Response(
         backup.to_csv(links),
         media_type="text/csv; charset=utf-8",
-        headers={"Content-Disposition": "attachment; filename=linkvault-links.csv"},
+        headers={
+            "Content-Disposition": f"attachment; filename=linkvault-links_{stamp}.csv"
+        },
     )
 
 
@@ -852,10 +855,13 @@ def export_sql(request: Request, db: Session = Depends(get_db)):
             select(Link).where(Link.user_id == user.id).order_by(Link.created_at)
         ).all()
     )
+    stamp = datetime.now().strftime("%Y-%m-%d_%H%M")
     return Response(
         backup.to_sql_dump(links),
         media_type="application/sql; charset=utf-8",
-        headers={"Content-Disposition": "attachment; filename=linkvault-links.sql"},
+        headers={
+            "Content-Disposition": f"attachment; filename=linkvault-links_{stamp}.sql"
+        },
     )