Subtask: Subscription data model and APIs

**Scope:**

- Design a normalized data model to support user subscriptions for Snapshots, Projects, Chapters, and Events
- Ensure the model is extensible to support additional notification channels in the future
- Implement APIs to allow users to subscribe and unsubscribe from supported entities
- Provide a mechanism to retrieve subscribers for a given entity, to be consumed by the notification layer
- Validate subscriptions to avoid duplicates and handle cleanup on entity or user removal where applicable
- This subtask focuses only on the subscription foundation and does not include notification delivery logic.

Part of #3227