feat(backend): record pool views to activity.pool_views

Add POST /pools/{id}/views, mirroring the file-view endpoint: it
enforces view ACL and appends a row to activity.pool_views (viewed_at
defaults to statement_timestamp(), so each view is its own history row).
The table existed but nothing wrote to it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/port/repository.go

@@ -129,6 +129,9 @@ type PoolRepo interface {
 	RemoveFiles(ctx context.Context, poolID uuid.UUID, fileIDs []uuid.UUID) error
 	// Reorder sets the full ordered sequence of file IDs in the pool.
 	Reorder(ctx context.Context, poolID uuid.UUID, fileIDs []uuid.UUID) error
+
+	// RecordView appends a view-history row (activity.pool_views) for the user.
+	RecordView(ctx context.Context, poolID uuid.UUID, userID int16) error
 }
 
 // UserRepo is the persistence interface for users.