Implement soft delete for tasks via new isDeleted flag

[MessiasLima]

Description

Currently, tapping delete on the task detail permanently removes the task. To support future synchronization conflict resolution and provide a recovery window, tasks should be marked as deleted instead of being removed immediately. The existing hard-deletion path must remain available for future cleanup.

Scope

• In scope: add isDeleted to Task, update the database migration with default false, add TaskRepository.markAsDeleted(id), wire TaskDetailViewModel.DeleteClicked to soft delete, keep existing hard delete methods.
• Out of scope: UI for permanent deletion, automatic hard-deletion cleanup, exposing deleted tasks to the user.

Acceptance criteria

• GIVEN a task exists
  WHEN the user taps delete on the task detail screen
  THEN the task is marked as deleted and remains in the database
• GIVEN a task is marked as deleted
  WHEN the app loads today's tasks or task detail
  THEN the deleted task is not shown (unless explicitly requested by a future query)
• GIVEN the codebase
  THEN TaskRepository exposes a markAsDeleted(id: Uuid) method that sets isDeleted = true and updates lastUpdatedAt
• GIVEN existing tasks
  WHEN the migration runs
  THEN isDeleted defaults to false
• GIVEN the codebase
  THEN the existing hard delete method remains available for permanent deletion