Deduplicate IOC identities and enforce uniqueness. Closes #1240

[cclts]

Description

Deduplicate existing IOC rows by (name, type) before enforcing uniqueness at the database level.

This change updates the IOC model to declare a unique constraint on ("name", "type") and adds migration 0053 to safely prepare existing data first. The migration keeps the IOC with the highest attack_count for each duplicate group, breaks ties by keeping the row with the latest id, deletes the remaining duplicates, and then applies the unique constraint in a separate migration step.

Related issues

Closes #1240

Context:

• Prevents duplicate IOC identities with the same (name, type) pair.
• Ensures existing duplicate data does not cause the new database constraint to fail during migration.

Type of change

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).
• Chore (refactoring, dependency updates, CI/CD changes, code cleanup, docs-only changes).

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[cclts]

Would appreciate a review when you have a chance @regulartim

[regulartim]

Hey @cclts ! Yeah, I saw that the PR was ready but did not find time to review yesterday.

[regulartim]

Looks good, thanks! :) I'll make a small simplification to models.py before merging.