feat: delete task/account, edit endpoint, richer event log

- store: DeleteAccount, DeleteTask, UpdateEndpoint (+ cascade/update tests)
- httpapi: DELETE /tasks/{id}, DELETE /tasks/{id}/accounts/{accountId},
  PUT /endpoints/{id}; delete guarded with 409 while task running
- orchestrator: enrich WS events with login/host/port/error (test + run + errors)
- web: delete buttons (task, account) with confirm, endpoint edit form,
  human-readable event log (source/dest, host:port, error text)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMHQTtnQtQqL8muAXHr9kd

internal/store/accounts.go

@@ -29,6 +29,12 @@ func (s *Store) CreateAccount(ctx context.Context, a Account) (int64, error) {
 	return id, err
 }
 
+// DeleteAccount removes one account (and its migrated_messages via ON DELETE CASCADE).
+func (s *Store) DeleteAccount(ctx context.Context, id int64) error {
+	_, err := s.Pool.Exec(ctx, `DELETE FROM accounts WHERE id=$1`, id)
+	return err
+}
+
 func (s *Store) ListAccountsByTask(ctx context.Context, taskID int64) ([]Account, error) {
 	rows, err := s.Pool.Query(ctx,
 		`SELECT id, task_id, src_login, src_pass_enc, dst_login, dst_pass_enc,