Add endpoint deletion

The endpoints screen could only create and edit servers, so a mistyped
or retired endpoint stayed in the list forever.

Tasks reference endpoints without ON DELETE CASCADE, so a referenced
endpoint is refused with 409 and a count of the tasks using it rather
than cascading away migration history. The foreign-key violation is
mapped to the same status to cover a task created between check and
delete.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/store/endpoints.go

@@ -26,6 +26,23 @@ func (s *Store) UpdateEndpoint(ctx context.Context, e Endpoint) error {
 	return err
 }
 
+// CountTasksUsingEndpoint reports how many tasks reference the endpoint on
+// either side, so a delete can be refused with a meaningful message instead of
+// surfacing a raw foreign-key violation.
+func (s *Store) CountTasksUsingEndpoint(ctx context.Context, id int64) (int, error) {
+	var n int
+	err := s.Pool.QueryRow(ctx,
+		`SELECT count(*) FROM tasks WHERE src_endpoint_id=$1 OR dst_endpoint_id=$1`, id).Scan(&n)
+	return n, err
+}
+
+// DeleteEndpoint removes an endpoint. Tasks reference endpoints without ON
+// DELETE CASCADE, so Postgres rejects the delete while any task still uses it.
+func (s *Store) DeleteEndpoint(ctx context.Context, id int64) error {
+	_, err := s.Pool.Exec(ctx, `DELETE FROM endpoints WHERE id=$1`, id)
+	return err
+}
+
 func (s *Store) GetEndpoint(ctx context.Context, id int64) (Endpoint, error) {
 	var e Endpoint
 	err := s.Pool.QueryRow(ctx,