Bug: Subscribers email field does not enforce uniqueness at database level

### Description
The `Subscribers` model does not enforce a database-level uniqueness constraint on the `email` field.

Although the subscription API checks for existing emails at the application layer, the absence of a uniqueness constraint at the database level can still result in duplicate subscriber records.

### Why this is an issue
Application-level checks alone are not sufficient to guarantee data integrity, especially in cases of:
- Concurrent requests
- Race conditions
- Direct database inserts
- Admin panel usage

This can lead to duplicate entries for the same email address.

### Affected code
```python
class Subscribers(TimeStampedModel):
email = models.EmailField(max_length=70)

Start by locating the Subscribers model and the subscription API's existing email check. Add and validate a database-level uniqueness constraint for email, confirming that duplicate subscriber records are rejected even when application-level checks are bypassed.

Written by the indexing model from the issue text.