Expose credential_count in IOC feeds and enable filtering by credential usage . Closes #1294

[drona-gyawali]

Description

This pr will provide credential-level visibility into IOC feeds by exposing a computed credential_count field and adding filtering support via min_credential_count and max_credential_count query parameters.

1. Queryset enhancement

• Annotate IOC queryset with:
  • credntial_count = Count("credentials", distinct=True)
• Enables per-IOC credential activity tracking.

2. Filtering support

• Added query parameters:
  • min_credential_count
  • max_credential_count
• Allows filtering IOCs based on credential interaction volume.

3. API response update

• credential_count is now included in JSON feed response.

Related issues

Closes: #1294
related: #1098

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