Add job detail page with activity log and cancel support

Operations are now clickable from the host detail page, linking to
/ops/{id} which shows the operation info, host link, duration, and
activity log filtered to that operation. Active operations can be
cancelled, which transitions the host to failed and releases the lock.
SSE activity events now include operation_id for real-time filtering.

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

internal/store/operations.go

@@ -61,6 +61,28 @@ func (s *Operations) ListByHost(ctx context.Context, hostID int64) ([]model.Oper
 	return out, rows.Err()
 }
 
+func (s *Operations) Get(ctx context.Context, id int64) (*model.Operation, error) {
+	row := s.DB.QueryRowContext(ctx, `
+		SELECT id, host_id, kind, state, COALESCE(image_id, 0), started_at, completed_at, COALESCE(error_message, '')
+		FROM operations WHERE id = ?
+	`, id)
+	var op model.Operation
+	var startedAt string
+	var completedAt sql.NullString
+	if err := row.Scan(&op.ID, &op.HostID, &op.Kind, &op.State, &op.ImageID, &startedAt, &completedAt, &op.ErrorMessage); err != nil {
+		if err == sql.ErrNoRows {
+			return nil, ErrNotFound
+		}
+		return nil, fmt.Errorf("get operation: %w", err)
+	}
+	op.StartedAt, _ = time.Parse(time.RFC3339, startedAt)
+	if completedAt.Valid {
+		t, _ := time.Parse(time.RFC3339, completedAt.String)
+		op.CompletedAt = &t
+	}
+	return &op, nil
+}
+
 func (s *Operations) GetActive(ctx context.Context, hostID int64) (*model.Operation, error) {
 	row := s.DB.QueryRowContext(ctx, `
 		SELECT id, host_id, kind, state, COALESCE(image_id, 0), started_at, completed_at, COALESCE(error_message, '')