Create SyncManager interface with NoOp and Supabase placeholder implementations

[MessiasLima]

Description

Cloud sync will eventually be implemented for premium subscribers. This ticket lays the groundwork by defining the SyncManager abstraction and its two initial implementations, wired to the user's subscription status so free users receive a no-op implementation by default.

Scope and constraints

• In scope: SyncManager interface, NoOpSyncManager, SupabaseSyncManager (empty), Koin provision based on subscription status.
• Out of scope: actual sync logic, Supabase realtime integration, task conflict resolution, soft-delete handling.

References

• Depends on Premium gating infrastructure #130
• Related spike: Spike: cloud sync design #155

Scope

• In scope: sync abstraction, two empty implementations, DI wiring gated by subscription status.
• Out of scope: implementing synchronization behavior.

Acceptance criteria

• SyncManager interface defines a start() method
• NoOpSyncManager implements SyncManager with an empty start() method
• SupabaseSyncManager implements SyncManager with an empty start() method
• Koin provides NoOpSyncManager when the user is not subscribed and SupabaseSyncManager when subscribed
• Subscription status is read from SubscriptionRepository.isUserSubscribed
• Existing app startup and DI tests still pass

Additional information

• This ticket intentionally leaves both implementations empty; the Supabase implementation will be built in a follow-up ticket.