Quick Add Magic with multiple labels (`*a *b *c`) fired all
`PUT /tasks/{id}/labels` requests concurrently via `Promise.all`. On
SQLite these overlap as read-then-write upgrade transactions, which the
busy_timeout can't resolve, so some requests fail with HTTP 500
("database is locked") and the labels are silently dropped while the
quick-add input gets stuck.
Expose a `concurrent_writes` flag on the shared `/info` response (true
for Postgres/MySQL, false for SQLite). The frontend config store reads
it and `addLabelsToTask` now branches: parallel `Promise.all` when the
backend supports concurrent writes, sequential awaits otherwise.
Fixes#2680
6e851e2ec2