Add non-nullable lastUpdatedAt field to Task entity and update creation flow

[MessiasLima]

Description

To support future synchronization logic, every task must store a lastUpdatedAt timestamp that is always populated and reflects when the task was last modified. Existing tasks must receive a sensible default during migration.

Scope and constraints

• In scope: add lastUpdatedAt to Task, update the database schema and migration, backfill existing tasks with creationDate as default, update CreateTaskViewModel to set lastUpdatedAt on save.
• Out of scope: sync logic, UI changes, exposing lastUpdatedAt to users.

References

• None

Scope

• In scope: Task entity, database migration, CreateTaskViewModel save flow, test updates.
• Out of scope: other ViewModels, UI display of lastUpdatedAt.

Acceptance criteria

• Task entity has a non-nullable lastUpdatedAt: Instant property
• Existing tasks are migrated with lastUpdatedAt set to their existing creationDate
• CreateTaskViewModel sets lastUpdatedAt to timeProvider.now() when creating or editing a task
• DAO and Repository tests are updated to include the new field
• App compiles and existing JVM tests pass

Additional information

• CreateTaskViewModel currently constructs a Task in onSaveClick() with creationDate = timeProvider.now(). The new field should be populated with the same value on creation and on every edit.
• The Room/KMP migration may require a default-value SQL statement or an AutoMigration depending on the current database configuration.