Add soft delete to executions

[tembo]

Description

Add soft delete functionality to executions to prevent permanent deletion

Changes

• Added deletedAt field to Execution model
• Updated routes to filter out soft-deleted executions
• Modified delete endpoint to use soft delete instead of hard delete
• Created migration to add deletedAt column

Want me to make any changes? Add a review or comment with @tembo and i'll get back to work!

[tembo]

Requesting review from @hkirat who has experience with the following files modified in this PR:

• backend/package.json
• backend/routes/ai.ts
• backend/routes/execution.ts
• backend/prisma/schema.prisma
• backend/routes/apps/index.ts