feat: deployment rollback to previous image

- Add previous_image_id column to apps table (migration 006)
- Save current image as previous before deploying new one
- POST /apps/{id}/rollback endpoint with handler
- Rollback stops current container, starts previous image
- Creates deployment record for rollback operations
- Rollback button in app detail UI (only when previous image exists)
- Add btn-warning CSS class for rollback button styling

fixes #71

internal/database/migrations/006_add_previous_image_id.sql

@@ -0,0 +1,2 @@
+-- Add previous_image_id to apps for deployment rollback support
+ALTER TABLE apps ADD COLUMN previous_image_id TEXT;