Implement two-way task sync in SupabaseSyncManager

[MessiasLima]

Description

Premium subscribers can keep tasks in sync across devices. This ticket implements the actual two-way synchronization logic inside SupabaseSyncManager, using the local Room database as one source and Supabase realtime as the other.

Scope

• In scope: observing local task changes, subscribing to Supabase realtime changes, fetching the latest version from both sources, resolving conflicts by lastUpdatedAt, writing the winning version back to both platforms, and creating missing tasks.
• Out of scope: UI for sync status, cleanup of old soft-deleted tasks, handling subscription gating (DI handles which implementation is provided).

Acceptance criteria

• GIVEN SupabaseSyncManager.start() is called
  WHEN the local task table changes
  THEN the updated task is pushed to Supabase
• GIVEN SupabaseSyncManager.start() is called
  WHEN a Supabase realtime change is received
  THEN the remote task is fetched and merged into the local database
• GIVEN a task exists on both platforms
  WHEN a change is detected on either side
  THEN the version with the later lastUpdatedAt is written to both the local database and Supabase
• GIVEN a task exists locally but not in Supabase
  WHEN sync runs
  THEN the task is created in Supabase
• GIVEN a task exists in Supabase but not locally
  WHEN sync runs
  THEN the task is created in the local database
• GIVEN a task is marked as deleted (isDeleted = true)
  WHEN sync runs
  THEN the deleted state is propagated to the other platform